I have a question on cassette sizing.

I bought a Bianchi for 40 bucks that a guy used to use
for crit racing. (The bike iD stamped on the head tube
is 0125633 -- I don't know what this means, or how this
would help me date the bike).

The back cassette has only big gears (no pixie gears for
heavy climbing). I want to switch the cassetes, but I'm
a novice about sizing. I measured the largest sprocket
on the cassette and it is 90 mm. The largest sprockets on
my touring bike are quite a bit larger, and I think that's
what I need.

My question: the 90 mm cassette doesn't correspond to
the sizes I see advertised. I see a cassette of 11-28,
12-21, for example, but I don't see how these relate to
90mm.

Thanks for any help.

Michael