I am trying to setup my Garmin 305 Forerunner + cadence sensor and am having some trouble. I am currently unable to ride on the road due to recent knee surgery so as a result I can’t have it auto detect my tire size (which is 650 x 25). This tire size is not an option as far as I can tell in the custom tire size setup options. Does anyone have any suggestions as to what I should set it to so that I can accurately track my distance while on the trainer? Thanks in advance!
Check your tyres are inflated to the pressure you usually ride on.
Put a mark on the tyre and the floor directly in line with each other.
Roll the bike forward until the mark on the tyre is has completed one revolution.
Mark the floor
Measure the distance between the two marks in mm
Do this as many times as you feel you need to, to ensure you have a good average number.
Insert that number into your 305.
When using the 305 on the windtrainer, turn off the GPS - press and hold mode until the sport options come up, scroll to the bottom until Outdoor is highlighted, press enter, change to indoors, prese enter.
Pure genius! 
Thanks!