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1)Those ties are fine for the trailer. Attach the bull snap to the trailer and use the quick release to tie your horse. That way, in an emergency, you can release the tie and free your horse. Ideally, those should release themselves when a certain amount of pressure is applied, but that doesn't always happen. They are designed, however, that no matter how much pressure is being applied to them, you can always release them simply by pulling down on them.
2) Use a safety release knot. Won't come untied by the horse pulling on it, but you can release it quickly by pulling on the loose end of the lead rope. This is a VERY important safety point. DO NOT EVER tie a horse with anything that you cannot release quickly in an emergency.
Also, if you are tying a horse to a trailer, make sure it is hooked up to a truck! A panicking horse can pull most small to midsized trailers over on top of them.
3)The lead rope should be tied with no more than about 1 1/2 to 2 feet of slack. If the horse can reach the ground, he can get his foot over the rope and get tangled in it. That said, your horse should always have access to water, so you should find a way to tie your water buckets up so that they are within reach when the horse is tied.

My other #1 trailering tip: NEVER tie the horse before securing them from the back!! Whether this is a "butt-rope" a "butt-bar" or closing the back door, make sure that there is a barrier behind them BEFORE you secure their head. If a horse starts to back out of the trailer after they're tied and hit the end of the rope, they can panic. A woman I used to ride with broke her leg in 3 places because she unfastened the butt-rope before she untied her mare's head. The mare started to back out, hit the end of the rope, panicked, and she was trapped inside. Someone was finally able to reach in and release the tie, allowing the horse to escape (thus the importance of safety releases!) but she was badly injured. Also, if the trailer is short enough that the horse can get his hind feet off the back before hitting the end of the rope, he might set back and startle when he hits the rope, and could slide his hind legs under the trailer and skin his cannon bones on the lip, which will just make him panic more.

Also, I know you have a straight load, so it's not really an issue, but for anyone with a slant-load, always teach the horses to back out! Allowing them to turn around in the trailer creates a dangerous situation for the handler, as it is a tight space. The horses are often also eager to unload, and can learn to bolt out of the trailer, which is extremely dangerous.

It is also definitely preferable to travel in leather or break-away halters as opposed to nylon or rope halters. If a horse should fall in the trailer (more common than you might think, I know at least two who have) the halter will break before the horse's neck, hopefully.

Haha, I'm here to help! I always figure the more help I give, the better off all horses will be. biggrin Not to say that your horses aren't already very lucky. You take wonderful care of them, from what I've read over time. smile
And you would be AMAZED at how many people don't follow the tying/butt bar rule. Seriously. So many people I see tying their horses with nothing behind them, or opening the back and then walking in to untie them.
And the lady that I bought my thoroughbred from actually TAUGHT him to turn around in a slant load. WTF? Seriously? It took me MONTHS to reverse that, and some pretty scary situations where I almost got pinned and trampled.

As for the length of the trailer ties, it's similar to tying. You want them to be long enough to be comfortable. To be able to easily reach any food that you're putting in there, and to be able to put their heads down so that they can clear their nasal passages (unless you have mangers, I suppose, then it would be hard to get their heads down... hm, I'd never thought about this before...). They should be short enough to prevent them from turning around though. It would be okay if they can move their head to look behind them, but they shouldn't be able to bend completely in half. Part of the reason for tying horses in the trailer is so that they can't reach over and pick a fight with the horse next to them.

Anytime you ever have a question, though, please don't hesitate to ask. smile I consider my primary responsibility as a trainer and instructor (after safety, of course, which is always first and foremost) to be an advocate for the horse. So many people (not necessarily you, like I said, you're pretty awesome biggrin ) mistreat their horses or take unnecessary risks out of simple ignorance. If I can make the lives of horses better simply by posting advice on some online discussion boards, I can sleep a little bit better at night. smile