Ninja UV will map to a specified texel density.. it will automatically scale based on the diffuse texture size or you can input a texture size. http://www.creativecrash.com/maya/marketplace/scripts-plugins/texturing/c/ninja-uv--2 you can also check out Ninja Dojo for more tools. http://www.bk3d.com/Ninja_Dojo/Dojo.htm
Well you can just select the shells you want and the scale tool will scale them all the same amount. If you want to get all your shells to have roughly the same texel density you can use the layout function in the polygon menu in the UV editor, or personally I use a free plug in called UV Deluxe.