What nib size should a beginner choose?

A Western fine or a Japanese medium — both lay roughly a 0.5–0.6 mm line. It is wide enough to feel the smoothness people buy fountain pens for, narrow enough to behave on ordinary paper and small handwriting.

Skip extra-fine for a first pen (more feedback, less forgiving) and skip broad unless your handwriting is large and your paper is good. You can change nibs later on most starter pens — the Lamy Safari swaps in seconds.