Does phimosis affect penis size?

Yes. As the penile skin forms scar it may prevent it from full erection. Early phimosis can be treated medically but if it's severe or doesn't respond you should consider circumcision.
Here are some ... Penile size will stay the same after marching into adulthood. Severe phimosis prevents glans penis to show off making the penis look deceptively shorter. If circumcision isn't done timely, chronic balanoposthitis will gradually cicatrize foreskin and penile skin to "hide & bury" the penis. As a man ages, erection lessens, ED may kick in, and eventually a smaller and shorter penis manifest.