Welcome to MobileRead, LadySavoy!
What you're encountering is the fact that the Reader's PDF emulator only does those two sizes -- the M is intended to crop the margins.
You'll likely find going forward that PDF is a dreadful format for e-books because its contents can't be re-flowed to accommodate different display sizes. That's the short version anyway. Search the forum for "PDF" and you'll find multiple very long versions.
In any case, most folks end up sticking with RTF (which sacrifices image display) or converting their files to LRF (the unsecured variant of BBeB), both of which work remarkably well on the Reader.
Again, welcome, and jump right on in.