The dropdown in Mail 26 only exposes preset sizes (10, 12, 14, 18, 24…), so typing 16 into that box snaps to the nearest preset instead of accepting an arbitrary value. Try highlighting the text in your draft and hitting Cmd+T to bring up the classic Font Panel — that one lets you type 16 as a literal size and it sticks for the highlighted text. If you want it as the default composition size, set the font first in Settings > Fonts & Colors using that same Font Panel trick, and Mail will use it going forward. Clunky workaround but it does actually work in 26.5.