I am assuming your bike has a 7 speed cassette - If I recall correctly, RX100 only ever came in 7 speed.

Bikeman Dimension Cassette - 7 Speed, 12-28t, Silver
Here is one example.

Basically, any 7 speed Shimano or Sram compatible cassette will fit on your wheel.

Since cassettes and chains wear together, get a new chain when you get the cassette. Chains come a few links longer than you will need and you have to take a couple links out when installing. Generally you want to size the chain so that it can go around the big cog in the rear, the big chainring in the front, and through the derailleur, + one link. There are other methods for sizing a chain and these are easy to find.

Short cage derailleurs were generally listed as compatible with up to a 27 tooth cog, and in my experience this is not a hard limit, so a 28 should work without issue.