I’m closing in on 60
guys 5 years and older than me are either ripping on boards the same height as they are tall or and inch or two bigger, or on 9’ plus high performance longboards.
I think you should always try and go either as short as you can or as big as you can but not in between.
Seems like at 5’8" riding anything 6’8" - 9’ creates problems regarding length and the style of riding you may want to do
Obviously the bigger the wave the bigger the board because there’s more face to work with but at the same sense it works that the smaller the wave the bigger the board as well as the bigger the wave the smaller and heavier the board (such as in tow surfing).
As long as the board paddles well and doesn’t bog going small can be an advantage.
You can take away some of the disadvantages of length with design and materials but eventually being oversized can create problems.
My smallest is currently a 6’2" x 21.9"x 2.875" quad but I hope to go down to 6’ or even 5’8" and maybe 22.5" wide or narrower and 3.25" thick
one of my favorite current boards is the board on the left below by Peter Poppler/Backdoor Kauai. Its a soft 8 channel quad stubbie with hand made balsa glass-on fins and around 6’4"x22.5"x2.75". You can pretty much do anything on the hybrid design from nose riding to heavy arcs and late drops. Wide and stubby no-nose boards feel like the future to me. I know allot of older locals riding 5’4"-5’8" Firewire Sweet Potatos and Tomo Vanguards. By my current firewire is a 6’4" potatonator at over 47 liters compared to a 9’ Fireflight 2+1 I had at 54 liters
https://swaylocks7stage.s3.us-east-2.amazonaws.com/s3fs-public/9508697_orig.jpg