How To Control Length Of Snare Sound?

How do you make snare hits sound "shorter"? Gate?

transient designer or volume envelope

Don't know if it is still included with REAPER but you could try the JS:LOSER/TransientController then turn down its "Release/Sustain" control.

Use reaGate. And then if your lacking some of that splash sound, set reagate or use some other device to send out some noise and make an artificial splash or tail sound. But generally a gate. If you only have 1 or 4 samples or something, I'd spend some time and put an envelope on it as bbb says and render them out to new samples. Saves cpu and hassle. Really, start making yourself some custom snare samples etc. Don't be affraid to mix and match. You may like the attack on a Roland 8 snare, but like the tail on some other snare. Mix em and render. ~Rob.