SRAM Rival-lower gears

[squarewheels]

Hi. I am new to this forum.
My present cassette is 12-25 and I need a lower gear in order to climb local hills (a case of old age and decreptitude). I would like to know what is the largest cassette I can fit so as to get a lower gear. My chain ring is a compact 50x34T.
Thanks for any advice.

[bfergie]

Certainly 12/28 but Sram now do a rear derailleur that pops on that will run a 32t cassette!!