Initially you could look for a 1/4 to 1/2 inch increase in both length and girth.
The "safest" pumps to get are those with a gauge, so you know exactly what you are doing. I think a great pump to get is the deluxe model from lapdist:
All Products | LA Pump
If you decide to buy, let us know your erect size and we can help with cylinder size selection.
The most effective pump routines are those that combine pumping and jelqing, usually several alternating cycles of each. See avocet8’s pumping 101 thread in the Pumpers Forum.
I like to put on a cock ring after pumping and do an erect massage. That would be similar to clamping, but I use an elastic cock ring as opposed to a hard clamp.
Remember to start slow and take it easy. You can never go too slow, but it is easy to go too fast.