I don't know how much space you have where the trailer is, or behind it, but I'd add on where you have the livestock trailer parked.
I would change the roofline and make it big enough for your tractor to pull in easy. If you have the space, I'd make it as long as possible. 40 feet at least, but maybe 60ft or more. It's something that you can do easily in stages and over time.
The beaty of this type of building is that if you build it a certain length, you can easily add on to it to make it longer!!! Just set more poles and start framing off the back. Take down the back wall and rebuild it at the end after building the extended walls. Theoretically, you could make it as long as you have land.
With an existing building, it's cheaper and easier to add onto what you already have then starting fresh on a stand alone building. Pole construction would be the fastest, cheaperst and easiest. If you keep the width so something simple like 24 feet or less, then the roof trusses are also very straightforward. That would give you plenty of room for a 16 foot wide door that you can make as tall as you want it to be. Measure your tractors height and add two feet. Then add another foot for the header and you will know how tall to make the walls. Taller is always better, but that should really be plenty.
Then if you find you need more room down the road, you could easily add on to it again by mirroring what you already have on the other side. When done, this will give you all sorts of room for storage and working. It's a design that allows you to slowly add on to it and make it about as big as you want.
I took your picture and erased parts of it to show what I have in mind and what I would do if it was up to me.
Good luck,
Eddie