A4 standard, A4 sport/S-line, S4, RS4.

I just measured my A4 2.0T S-Line.
32mm Front
18mm Rear
Looks like they're pretty well powder-coated so the actual bar might be 1mm thinner.

No idea on the front ones though.

Also looks like standard suspension is a 16mm rear bar.

A4 non-sport...16mm

A4 sport...18mm

A4 S-Line...18mm

S4...20mm

RS4...22mm