I'm a guy and I love big dildos and real parts! That's the short answer.
I've only been with one guy and his penis was average size (but with a wonderful curve and a pronounced head). I have always wondered what being with a guy with a larger penis would be like, but it's really hard to imagine anything more wonderful than what I experienced with my friend.
I used to think
I might be a size queen with dildos, because after starting with a rather small toy, I definitely enjoyed average and above-average ones more. But after trying some really large ones, I realize I'm a 'just-right' queen! The Cyberskin Extreme Cock proved too large for me to really enjoy for oral; the Vixskin Outlaw is probably too big for me to enjoy being pegged with. I prefer toys between 1.75" and just over 2" in diameter for oral and between 1.5" and just under 2" for pegging. Length is less important, but anything up to 8" or so is great for oral. Your personal preferences may be different!
You can't go too wrong with Vixskin Johnny, in my opinion. This is my favorite dildo for pegging and it's also excellent for oral. It's big, well above average, but not so huge to be majorly intimidating. I definitely would recommend Johnny over Magnum because of the material. Regular silicone is no more user friendly than Vixskin and it's usually A LOT firmer, which in my experience just doesn't feel as good as something close to normal human texture. Firmer dildos also feel considerably larger than more flexible dildos of similar diameter.
Have fun! And let us know what you decide.