The vagina only expands so far depth-wise, so depending on how large your partner is the whole penis may or may not go in. If he's larger than 6.5 inches or so the whole penis will probably not go in. I don't think you need to be worried or overthink things; if you both are prepared properly it will probably happen pretty naturally.

I want to emphasize the use of lube, though, especially if he's large. There's no shame in needing extra lubricant; it's there to enhance pleasure. Be sure whatever lube you use is compatible with condoms if you choose to use them (which I recommend). Also, PLEASE be sure you have proper contraception before having sex! In my opinion it's best to double up (condoms + the pull, the shot, the ring, other method, etc.) but that's a personal decision for you and your doctor to make together. I just strongly recommend waiting until you're on birth control.

Good luck, and have fun! If you have any other questions please feel free to PM me.