How your penis will look in porn depends on how big your penis actually is and the size of your body. The vast majority of porn is an optical illusion...short, skinny guys with above-average sized dicks (6.5" to 7.5") and average girth (4.75" to 5") all having sex with tiny women with small hands. On video or in pics, girth counteracts length. So, basically, a thick cock always looks shorter than it actually is (hence the need for these porn actors to not be too "girthy").
I liken it to bodybuilding. I was once working backstage at a bodybuilding contest and was amazed at how small (short/lean) these guys were in clothes. However, once they took their shirts off, they looked a lot bigger. Put them up on stage with the lighting, the fake tans, all oiled up, and they looked like they were a foot taller and 100 lbs. heavier. A total optical illusion.
To summarize, your 7"-7.5" penis will look large on film (i.e. equivalent to large cock pornstars) as long as you're not taller than 5'8" and don't weigh more than 150 lbs.