Make sure its 128 - 256 - 512 - 1024 in size, square. ( Chilli skinner will usually make the space almost a perfect polygon, but you need it square so move some around till you've got a tight square area. ) 512 is as big as I would go personally, though I havent got a skinned model into Generals yet as I lost my project with a harddrive failure

. But the details looked good in RenX and the W3d Viewer at 512 x 512.
Then save it as BMP, JPG, or TGA ( all bitmap types ), and use the material editor to apply the skin to the model.
Save a copy as DDS with the DDS Plugin to make a professional job of it before Distribution or inclusion of a mod.