From a brief google I find the following
from this thread on the TDI club forums
Bores and comparisons
Garrett designated turbos--------- intake Bore-------------- Exhaust Bore
GT 1749 V (GT15) ------------------1.326" -------------------1.441"
GT 1749 VA-------------------------1.390"--------------------1.467"
GT 1749 VB (THE"17) ---------------1.459" -------------------1.505"
GT 2052 V(The "20) -----------------1.550***---------------- 1.550***
GT 2252 V---------------------------1.550 ***----------------***
GT 2356 ----------------------------1. 587 --------------------1.605
GT 2552 V-------------------- to many sizes to list ------------1.550 to 1.700
*** These sizes vary in different series within that particular family group. Example, there are over 100 variations of the VNT 20 alone. The sizes shown here are of the most common sizes that are used in turbos manufactured for use in VW applications.
GT1749V is the "VNT15" turbo.