The ideal size for you is primarily dictated by your all up flying weight (including the reserve parachute).
Just like wings, reserve parachutes have weight ranges. The more weight you load it with the faster you come down. As well as a high sink rate, an overloaded reserve also has a higher risk of down-planing and oscillations. So it’s important to look at various brands and styles to make sure you’re choosing one where you’re in a good place in the weight range.
It’s not recommended to have a higher sink rate than 5.5m/s as the chance of injury on landing is high. This is the limit for certification under the EN 12491 system.
Many pilots have a reserve that is too small. This happens because smaller sizes are usually cheaper and lighter. However, once you’ve seen a pilot deploying a small reserve you’ll want to get rid of it (even cheaply), and so the cycle continues. Don’t be part of the problem, get a big reserve to start with!
Opening speed is the argument often quoted to support smaller reserves, but the time to reach stabilised descent can be much longer due to the down-planing problem that a small reserve creates. Real life opening speeds vary greatly from the controlled drop test shown in EN reports. The design and packing can influence the reliability much more than size.
A reserve should not be too large, as this might cause oscillation problems. For this reason manufacturers usually quote a weight range, although this is not part of the certification testing. Note that sink rates quoted on certification tests are in perfectly still air after disconnecting the paraglider using quick outs, which very few pilots are equipped to do. Your real sink rate might be higher if your wing causes oscillations or down-planing or you’re in sinking or turbulent air. The attached (stalled) paraglider also slows any ‘gliding’ style reserve (square, Rogallo) so might increase your sink rate. Your sink rate might be lower if you have gained control of your flailing wing and it is assisting the PDA or SQR reserve.