I’ll get this out of the way first, you don’t need a DJ bike to learn to jump/pump/trump. Most hardtails will do (as long as they have a fairly low standover). Just ditch the gears, slam the seat and set the forks up hard.
However, this being where it is, you probably have £2k to spend on a jump specific bike 😆
Size wise they are all pretty much the same. Makes no odds as they’re for jumping and not for sitting on 😀 . And don’t get sucked into all this geometry crap too, it makes eff all difference.
Steel or alu frame? – makes little difference. A light cro-mo frame feels better but costs more. Cheap steel is heavy but strong, cheap alu is fine (especially if you’ve never rode a good cro-mo frame)
There’s loads of mid-level bikes circa £500 that would all see you right. Weirdly some have gears, which you don’t want or need so if you get one of those, ditch the gears. Front brake isn’t need either. You want to strip off all the crap you don’t need to save weight and have less things to get tangled up with/break.
Forks don’t need to behave like a trail fork so forget plushness etc. Firm, one hit wonders are fine (you can use 80-100mm xc/trails forks adjusted to suit if you are smooth/t’awesome). Flat pedals obviously, pump your tyres up hard and that’s about it.