Sildenafil, the generic form of Viagra, as well as Tadalafil, the generic form of Cialis, all work roughly the same way. They dilate your blood vessels to allow more blood to reach your penis which then helps create an erection when you are mentally and/or physically stimulated. CAUTION: Do NOT use poppers when taking these meds because when combined, it can cause a drastic drop in blood pressure.
No Erectile Dysfunction drugs increase the size of your penis, as that is not what they do. As
@beachforme says above, ED drugs can enable men with erection issues to get erections.
Are ED drug erections firmer, harder? Yes, they can be.
Do ED drugs "max out" your erect size? Yes, that can happen too, but you are no larger than the maximum size your erection would be without using the drugs.
I've mentioned in the past trying full strength Cialis (20 mg) for the first time. For me, I got very quick erections, hard and stiff enough to hang a bath towel on, and when I measured myself, I was at my maximum full length and girth, but I was no longer or girthier than I was at 18!