The GAP (inside distance) between the Two Legs of the Matching
Transformer is normally less than you can achieve using standard
plumbing fittings especially on the upper frequencies. So the
measurement is often doubled so to speak from 1/20th wavelength to
1/10th wavelength.
When increasing pipe diameter, don't forget the K-factor!
The 1/4 wave matching transformer will be slightly shorter, the 1/2
wave vertical will be shorter, and the GAP will be very slightly
narrower.
Since you cut one leg of the transformer in conjunction with the
vertical element as one piece, don't forget to shorten this pipe by
both reductions in length.

TTUL
Gary