I would think 12-15"; maybe look for one that's wider than it is deep.

R1-1012 Stainless Steel Bar Sink, 18-Gauge · More Info

ALFI AB1218 Small Rectangle Fireclay Undermount or Drop In Prep / Bar Sink · More Info

There seem to be plenty of options that would fit the bill -- I just searched here for "bar sink"...

We have this one in a "bisquit" color. It's drop-in or undermount.

https://www.amazon.com/Blanco-440204-15-Inch-Diamond-Sink/dp/B000LWD93C

Kohler. Assuming the sink base is at least 24 inches wide. Front to back a little over 11 inches, allowing room for faucet on counter , installled sideways

Installation:
Min. base cabinet

width:
Bowl area (Only):

Drain hole: Template:

Single Under-mount 15" (381 mm)

Length: 9" (229 mm)
Width: 15-3/4" (400 mm) Bowl depth: 5-1/2" (140 mm) 3-5/8" (92 mm)
113194-7, required, included

https://www.us.kohler.com/us/Undertone-10-3-4-x-17-1-2-x-5-5-8-small-squared-under-mount-single-bowl-kitchen-sink/productDetail/kitchen-sinks/418216.htm?skuId=358244&brandId=432917