I’m looking for a jump bike for my 11 yr old son. What size do I go for? I have no idea what the sizing is relative to a MTb. He will want to ride trails and mess about, no long days out because kids stop when the fun stops. Any advice? Looking at a kona stuff but only comes in 13.5 inch

Depends how tall your 11 yo son is.

But DJ bikes are generally one size fits all

top tube length is most important, providing the standover is fine, which it should be on a jump bike. Try putting the seat down very low on a regular mtb for a guide then check out the dimensions. two ways of measuring tt – actual and on a horizontal plane. Should give some idea anyway. Best way really is to try a few tbh, even if you just sat on a few in showrooms to get a feel.Also check the seat tubes angle – in a slack seat angle – a seat raised six inches can really make the cockpit alot longer, and thus suit someone taller, but not so good for someone short who just wants to put the seat up to ride a bit of xc to get to the jumps – although fine when the seat is slammed. Surely kona have some height / size guides online??

Depends really, my 11yr old is 5’3″ and rides the same size DJ frame as me 14″(22″ TT) but that’s as a general mtb whereas mine is actually used as a jump bike (I’m 5’11”)

Most decent DJ frames actually come in long or short sizes, ideally for anyone under 5’5″ you want the shortest TT and stays you can find sub 21″ and sub 16″ and if you find more than one or two bikes to fit the bill, the lowest standover out of them. Any shorter than that height and 24″ wheels probably make a lot of sense.