Making a pixel texture for a weapon that is the right size.

I'm making a mod currently called Blades and Bows and I have a problem.

Its not a huge one but I want to know how I can make a pixel texture that is the same size in hand as the texture.

Example: I make a spear texture and its 21 by 21, I add it to a weapon and open the game, then in game the weapon has the 21 by 21 texture but in hand is the same size as a normal sword.

I know I can make a model and it will have a different size but, I kinda like Minecraft's pixel textures.