What format should clips be for good AE performance?

If the dimensions "non-standard" it's pretty much irrelevant, as this will disable most acceleration functions, anyway. Something to do with the underlying math. If you really want to be sneaky in fact you wouldn't use clip formats at all but rather rely on image sequences. AE still loads those natively in many formats and in the day and age of fast storage even bulky 4k TIFF or EXR sequences can be played that way in realtime without extra caching a lot of times....

Mylenium

If you are going to do further processing pick a visually lossless production format. There are several options in the Adobe Media Encoder. Unless you are an expert in compression stick with the presets.

Same thing for comp size. There are standards for frame sizes that should be followed. You can't have odd numbers of rows or columns in a frame size so 971, 305 won't work. It will be converted to the closest standard frame sizes for MPEG compression if you choose to render it to the most common H.264 MP4 format. Here again, don't be messing with the presets unless you really really really know what you are doing. You'll just get frustrated.

All social media platforms have recommended frame sizes and frame rates for their platform. Most of them have presets in the AME.