new to surfing any help

hey guys and gals, im new to surfing and dont really know that much about it i have been a couple of times but am having trouble trying to work out what size board to ride as i have not court any type of wave yet, my pop up and balanceis okish, i do need to work on it but everyboard i hire seems to sink and i dont know whats going on, do i need to to go bigger than a 7'10" or smaller.
if anyone can help i would be well greatful as am going surfing again in a couple of weeks and really want to catch my frist proper wave.
thanks

if anyone can help i would be well greatful as am going surfing again in a couple of weeks and really want to catch my frist proper wave.
thanks
