Our arena is quite small too ( Maisys 15.3hh but rides big! ) And its a struggle to really do alot of jump work in our school, but usually i try to have jumps on the diagona- this gives you the full legth of the school!
But it has its pros too- It learns you to sit up and think about where you are going! This is great for practicing balanced turns and accuracy!
I rememeber a few years back i was riding my first horse at hom, Tommy, a 14.2hh Coloured cob (who could be quite strong when jumping) we jumped a single fence in the school and he ran on and jumped the school fence! Since then ive always had that in the back of my head!
I struggle in our arena, i can get a double down one side a scary filled one up the other side and then a skinny across the diagonal, i cant jump across the width of the arena as its just to tight for my boy but the smaller horses/ponies can.
Make sure everything can be jumped from both directions so it gives you more options.
Our school is pretty much the same size. We normally put a "cross" of for on the diagonals, a couple on the long sides (these can be related distances, bounces or whatever stride you want) and one on a short side.
E.G. you'd start at A on a right hand, jumping two along the long side, then go past c, right into the corner, turn onto the diagonal, jump the four there as a figure of eight (coming back to the same corner), then carry on to a couple on the next long side and the finishing jump near A
Depending on how many we have out and what we want to work on we may put some up to be jumped both ways..., e.g. you'd start as above, do your figure of eight, then the other two on the long, then back into the figure of eight after A ...
I am currently working with 2 very different horses, both 16.1, 1 is very experienced the other very green, they often work together when jumping as can both benefit from similar exercises.
I can build a grid, bounce to a one or two stride fence or 3 on one stride easily yes you need balance and control but you are aiming for that anyway.
Often I put one fence on the straight with 2 on dog legs, they can jump in any direction and help with focus and straightness.
A few weeks ago I had a skinny at x as part of a short course, even the baby horse was able to lock on and do this on a fairly acute angle, I was very impressed with him.
The more challenges they can deal with at home usually benefits them at comps, the young horse has never stopped and is now being placed with steady double clears, he is not ready to go against the clock but is well balanced so cuts a few corners and is getting very clever.
20x40 and up one long side cross pole, stride, upright, stride upright, spread over x on diagonal - or another uprigth if want to jump both ways and then double with varied pacing on other long side. Makes jumping anyhwere bigger really seem a bit easier, plus ours is deep surface and my horse prefers to ping off a less deep one!
We have a small indoor school and everyones always amazed how I build a full 8 jump course in there. You should try drawing a few ideas and think about making the jumps close together but the space between them as big as possible, make sure you use all of your corners etc. So the jumps can touch each other and lead off of each other but make it so as you must travel around the jumps in a sweeping line... And think about the flow. Once you start trying to put full courses in and use the space it's suprising how much room you have and how much acceleration you can build up!