I am not a professional rod builder, and there are certainly others on this site who know alot about this.
I can however offer what I have learned about the subject of line guide sizes.
Line guide size is determined by inside diameter. Each different ID is issued a size number. For instance, a #3 guide is listed at approximately 6.1 MM inside diameter, (which is a horizontal measurement.)
A no. 2 is smaller @ 5.6 MM, and a no. 4 is
larger @ 6.5 mm.
The converson factor for MM to inches is .03937, inches to MM is 25.40.
The above guide sizes are approximate, as each company may have slightly different sizing.
Good luck, Panfan