The 41 cm is really for training sails and also if you are sailing in areas with shallow sandbars
or rocks etc. It handles sails up to around 6.5 m2 fairly well.
The 52 cm is a much better (higher quality foil) fin and will handle sails up to around 8.5 m2.
Also, I believe your Go171 may have a center fin box with a plug.
In this case use the larger fin in the center fin box and the 41 cm in the rear fin slot if you are
an advancing beginner and need the additional directionality and rig steering ability to stay