There's a 250/305 FSM pdf available on the internet (green cover), but the diameter won't be listed if you need that.
The Fork Spring Free Length chart is on page 152. Standard = 452mm. Service Limit = 442mm.
There's also a fork oil chart on the same page. 150cc. 440mm from the top of the inner fork tube.
note: as per the instructions, check the fork oil level w the springs removed.
Page 112 says to install the fork springs w the smaller diameter end facing down, so that end goes in first.