I just realized that if my Dex-based Magus uses MP2 to become tiny, she loses her 5' reach and can't threaten anyone with her tiny scimitar, which goes down to 1d3 dmg (no big deal). Sure, she gains +2 attack & dmg from the Size bonus to Dex (w/ Wpn Finess & Dervish Dance). So it looks like MP1, going to small size, is the best I'll get out of the set of MP spells. By the time you can cast these spells, you've already bought DEX enhancements, so Cat's Grace goes out the window. At least, with MP1, I still have 5' reach and the +2 DEX bonus from size.
Am I wrong about any of this??
I just realized that if my Dex-based Magus uses MP2 to become tiny, she loses her 5' reach and can't threaten anyone with her tiny scimitar
You are correct about tiny creatures.
Tiny, Diminutive, and Fine Creatures: "Creatures that take up less than 1 square of space typically have a natural reach of 0 feet, meaning they can't reach into adjacent squares. They must enter an opponent's square to attack in melee. This provokes an attack of opportunity from the opponent.... Since they have no natural reach, they do not threaten the squares around them. You can move past them without provoking attacks of opportunity. They also can't flank an enemy."
However, Monstrous Physique doesn't specify that your reach changes. But it's a transmutation (polymorph) spell.
Polymorph explains more about those abilities in general. However, it doesn't mention natural reach.
I think it's generally assumed that when you change size (for any reason) your natural reach may change also. Perhaps that's covered under "assume the form" but it's not spelled out anywhere I can find.
Well, if you want to be conservative about it, always look at a creature's entry to determine what reach you have with your attacks (sans any abilities you can't take on with your MP spell, like an ability that allows a monster's tentacle attacks to reach out further, for example). If it doesn't specify a reach, just determine reach by size.
Note that being tiny also halves your non-magical armor and shield bonuses (enhancement bonuses; mage armor, shield spell, etc... still protect at full strength). Since the text says nothing about rounding up (IMO, it should, or at least shouldn't be able to reduce something down to 0...) the default rule of rounding down would apply. So if you use any physical armor or shield at all, you'll likely lose at least 1 point of AC from it.
The game system is really rough on martial smaller folk. Whether it's finding gear appropriately sized from what the NPCs drop, or discovering your gear costs the same despite needing less materials, midget warriors are constantly getting shafted. :(