130mm for the two biggest, 74mm for the smallest.
Edit: If you can't see that pdf, I'll try a link to the Shimano Techdocs site.
Edir Edit: Are all Soras 8 speed?
No you get 7/8/9 sp Sora's, I belive the triple 9sp has a different inner BCD, just checked no all are as far as I know 130/74, I had a prowheel with my Sora 3300 setup it had a really odd inner BCD.
Sora Range as far as I know 7/8sp 3300, 9sp 3400 (thumb shifter), 9sp 3500 (paddle shifter), most of my Sora kit was 7sp and has 3300 designations, but it could just be it was 8sp with 7sp Shifters, which I will need to look at but sure they were Sora.