Quick gearing question - SV2

[theChemist88]

So I should be picking up a SV2 on weds eve, and im trying to get it all ready to run friday night (electronics should arrive in the post today). One mistake I've made however is I dont have any pinion gears!
So two things, 'roughly' what size should I start out at when using a 9.5T bl and 83 spur and if I pick a couple up before the night but dont end up with the right sizes can anyone lend me a more appropriate sized one on the night pretty please