rough estimate is to divide x ft by 3 and then take a little bit less ...
ie. 30ft jump is 9-10m
or use a converter:
also keep in mind that there are different types of jumps, tables, step ups, step downs, etc.
personally i'd go for the biggest kicker you feel completely confident on and you're able to throw most of your current tricks on .....
what double are you thinking of throwing first ?
last season a friend of mine did his first double back on a BC kicker of about 30ft / 9-10m ....