Erfurt did make small ring '98a, called carbines, even though they had a 24" barrel ( straight taper, no "steps" ); not be confused with earlier carbines with short 18" or so barrels.
Easiest way is to look at the left reciever wall ( next to magazine ). If it is the same diameter as the reciever ring ( where the barrel screws in ), then it is the small ring.
If the diameter of the reciever ring is larger then the reciever wall, then it is a large ring. The difference in diameter is quite noticable.
FWIW, I have mixed feeling about the strength of the small ring '98's. On one hand, in the Frank DeHass book ( bolt action rifles ) he states that the small ring '98s are best suited for lower pressure rounds then the '06 or the full-house loads of the 8X57.
On the other hand, the much-sought, fabled G33-40 ( along with the Czeck version, before Nazi's invaded ) was a small ring '98, and they are used as the basis for very high-end custom rifles, in typically '06. 270, etc.
Mark