What size will they grow to between next week and may?

If I started seeds next week (let’s say durban poison or bangi haze for arguments sake) what size would they grow to, if kept in vegetative state, by the time I plant them outside in may?

Way too many variables. Depending on genetics, pot size, feed schedules, media, lighting, etc... you could and should have some trees by then. That said, you could take that time and really train and shape them to fit in any space.

Something like this

You could start one and keep it cut back and get some clones established bf they’re ready to go out. Then flower the mother inside.
Also just not too early in May as your plants may go into flower without supplemental lighting.

I grew a Bangi Haze outside June-Oct, It was over 10 feet...