There have been very many studies done over the years to determine if horses prefer to travel in a straight load trailer or an angle load. Different studies result in different conclusions. However, a careful look at the studies from independent groups who used carefully designed protocols shows that overwhelmingly horses prefer to travel at approximately a 45 deg angle to the line of travel. Most studies that show horses liking straight load are poorly designed and performed by vested interests like float manufacturers.
About 30 years ago a Canadian study by a government research body measured stress hormones in horses during transport. They concluded that horses showed the least amount of stress when facing backwards and at an angle. The next best was facing forward at an angle and the most stressful was facing straight with the line of travel. Several other independent groups have confirmed these results over the years.
However, the problem with angle load floats in Australia is that the law limits the width of a float to a maximum of 8ft (2.44m). For most horses over about 15.2hh (1.57m) standing at 45 deg, it means that length of the bay area is too short to give sufficient space between the wall and his nose on one end and the wall and his rear on the other end. Horses can become quite troubled about bumping their nose on the sides of the float. Many horses travel with the head very low to prevent running into the wall with their face.
The way around this is alter the angle of the bay from 45 deg to make the length of the bay much longer. This may mean that your 2-horse angle load float becomes a single horse trailer and your 3-horse float becomes a 2-horse transporter. You can also remove the dividers that separate the bays and allow the horses to choose the angle that's most comfortable for them.
I have already sort of covered the length of the bay in the angle load float. But by far the biggest hurdle issue when it comes to dimensions of float is the interior height.
In Australia, the height of the inside of a float can very between 6'9" (1.93m) and 7'2" (2.18m). In my view any horse over 15hh (1.52m) needs a float with an interior height of at least a 7'4" (2.23m). Add an extra 4" (100mm) to the height with every 4" (100mm) in height of the horse.
Some floats are sufficiently high for a horse, but the manufacturers often place a lip than hangs down from the edge of the roof at the entrance to the float. I saw a float recently that was 7'4" interior height, but there was a 3" lip that hang down as the horse went to duck his head to enter the float - making the height at the entrance 7'1".
I know this is sometimes done to strengthen the structure, but there are many horses that are bothered about the risk of bumping their heads at the entrance of the float. I would not buy a float where this was a problem.
There are many different suspension designs available for floats. In Australia, most floats use either coils or a leaf spring system. But in my opinion torsion suspension is preferred. The ride for the horses is far superior. You can also get air suspension customized to your float, but it can be quite expensive.
If you have a choice, always choose electric brakes on all four wheels. Install a quality brake controller in your car that can be adjusted for weight of the trailer and road conditions
Good ventilation is essential. The average 2-horse float (with 2 horses inside) requires a complete change of air approximately every 7 minutes to keep gases, humidity and temperature within comfort limits for horses.
For this reason I don't like fully closed floats with storm doors. I prefer the back of the float to be open. I also prefer open sided floats that have slats rather than windows.
It doesn't matter. But if you are to use a step up system it is a very good idea to have a heavy-duty rubber bumper on the entrance to the float. If the horse slips off the float is prevents the leg from being grazed.
I do want the door to be easily secured. Some floats do not have safe mechanisms that ensure they can't open during travel. I don't like the old fashion arm that swings down and locks the ramp shut - these are notoriously unsafe despite being the most common.
I believe it is essential to have access doors into the float. Each horse should be accessible without having to enter the horse bay. If a horse has lost it's balance or is scrambling or fallen, it's important that you don't expose yourself to risk by being in the bay with the horse as you try to help him.
Many people like the non-steel constructed floats because they feel they are lighter to tow. But this is not always the case. It's wise to check and compare the weights before buying.
I prefer steel construction from the point of view that it is easy to repair. I know it can rust, but rust proofing can reduce the problem and even when it does rust repairs are usually simple.
Aluminium repairs require a specialized welding and fibreglass or plastic construction also needs expert repair people.
However, some floats have aluminium flooring and I like this over wood flooring because it is long lasting and resists damage caused by urine and moisture.
A horse float should be light and airy. No horse likes going into a dark hole. Other considerations are safety, which I haven't really discussed in depth. Look for overall quality of construction and make sure there are not things that stick out from the float that could injure a horse. Check there are no sharp edges on mudguards or areas that a horse could stick a foot into around the hitch. Tie-up rings should be strong and safe. They should sit flush with the float when not being used. Door handles on access doors should be recessed and not stick out.
There are many other aspects of floats that I haven't mentioned, but I hope this gives you some thoughts if you are looking to buy a float. Not everybody is going to agree with my preferences, but if you do your research carefully you'll make the right decision for what works best for you.