Personally I’d get the biggest one you can get, I don’t you’ll ever think “I wish the shed was smaller” but I wouldn’t be surprised if you ever think “I wish I had more space”! If you been ‘permitted’ 4x3 by ‘the boss’ I’d use it!! You’ll fill 4x3 very very quick with a few bikes, spares, wheels, tools, workbench etc.
Depends how handy you are but you can build something that’s considerably sturdier and more substantial for the money than the ‘shed kits’, for the same money. My farther in law recently built a behemoth of a shed, stud construction, fully insulated, ply lined etc. It’s basically a small house, I think he speant £1500 odd on materials - which is a lot - but not when you consider the size and quality of the thing. There’s a tonne of stuff online about building your own if it’s a route you’d consider, I realise it’s not everyone’s cup of tea though, sometimes you just want the easy option and to have it up and ready to use as quick as possible. If you’re going down the wood ‘pre-fab’ route make sure it’s pressure treated, most are but not all.