A shaving brush marked 26mm or 28mm is describing its knot diameter, not the width of the handle or the full spread of the tips. Both sizes can build enough lather for a shave. The useful question is how much brush you want on your face and how the complete brush fits your hand.
What the size tells you
The knot is the bundle of fibres or hair secured in the handle. Its nominal diameter gives you a starting point for comparing brushes. A larger knot has a broader base, but it does not tell you the exact working width once wet. Fibre type, knot shape, density and the amount of exposed hair all affect how it opens under pressure.
That exposed length is usually called loft. Two 28mm brushes can feel quite different if one has a shorter loft or a more densely packed knot. A maker's stated knot length may describe the complete knot rather than the installed loft, so check what is actually being measured.
When 26mm makes sense
Consider 26mm if you prefer a more contained application around a moustache, sideburns or a shaped beard. It can also be a useful starting point if a large brush tends to spread lather beyond the area you intend to shave. You can still build lather in a bowl or directly on your face; the diameter does not dictate the method.
The Alpha Bandit lists an Alpha G5 synthetic knot at 26mm. Its handle is a separate part of the comparison: assess the grip and shape as well as the knot rather than choosing from the number alone.
When 28mm makes sense
Consider 28mm if you enjoy a broader brush and want to cover a large shaving area with fewer painting strokes. A larger knot may need a different soap load from your current brush, but it does not follow that every 28mm knot consumes more soap than every 26mm knot.
The BetaHive USMC Red pairs a 28mm Alpha G5 synthetic knot with a beehive handle. The Outlaw Titanium brush also lists a 28mm knot, with G4 and G5 choices. Their different handles mean these are comparisons of complete brushes, not a controlled test of diameter.
A practical way to decide
Measure the brush you already enjoy, or note whether it feels too broad or too narrow.
Compare knot material and loft alongside diameter.
Check handle height, shape and weight separately from knot size.
Allow enough room in your lathering bowl for comfortable movement without striking the rim.
Check whether the selected option includes a fitted knot or is a bare handle.
If you are undecided, choose around a specific need: more precise application points towards the smaller working brush; a broader feel points towards the larger one. Avoid pressing harder to make a brush spread. A comfortable handle and a knot that opens easily under light pressure matter more than selecting the largest number.