I'm currently on Alza's 15" rim with PilotSport 3 195/55 R15 on my Myvi 1.5. The tyre provides good traction on both dry and wet, can layan corner with confident.

For comfort... suggest you stick with stock rim size 14" 175/65 R14 or change to 185/60 R14 also can. (wider tyre, better traction)
For looks, can go for 16" but you can forget about comfort edy... haha.
If it is between comfort and stability, can go for 15" with 195/55 R15.
If only for stability, can go for 195/50 R15 but your car will be bumpier.

Just my 2cents.

For looks, it really depends on your own preference. You may try to find some pic online on Myvi with 16" & 15" so you can compare both of them. .
For comfort, previously I was on 195/50 R15 and it was bumpy and now it is better with 195/55. If it's on 16'', the tyre size should be 205/45 R16, it is wider and ''thinner'' so definitely it will be bumpier compared to 14'' and 15''. If you can live with it, then 16'' is your choice. Also, 16'' tyre is more expensive compared to 15''.