That really depends on where you are now. Before considering injecting, you need to get your testosterone tested. From there, you have a baseline.
My baseline was low. Doc put me on clomid. Balls grew quite a bit. Clomid stopped working after two years. Went on TRT, balls did shrink, but I wouldn't say it's noticeable except to me. If you want to retain fertility (I'm done having kids) you need to be on a low dose of HCG.
I haven't gone super high level, but just going from bottom of range to top of range is life changing. Aside from the physical, the confidence boost is incredible. I solve problems now effortlessly. A problem comes up, I think about it, the answer becomes clear, I take action, done.