If your shapeshifting doesn't break thermodynamics, then you're not able to shapeshift into anything smaller or larger than yourself.

(Though, perhaps, you could stretch it a little by 'cheating' it -- become larger by shifting into a hollow shape filled with air, become smaller by shifting into a shape filled with very dense material. But there are limits to that -- too big, and you'll float away like a balloon; too small and the ground/floor beneath you won't be able to withstand the pressure, so you'll sink down into it -- and you'll never be able to change your overall weight/mass, which might compromise your disguise in some situations.)