You can save in any size you like. Points to be born in mind are:-

• Textures tend to be made at the same size as the template. If you want a highly detailed texture then you'll need a larger map. As a rough guide I use 512x512 for general scene props, 1024x1024 for average resolution body and head textures, 2048x2048 for higher resolution body and head textures.
• Some programs prefer square textures, some also want the sides to be in powers of 2. i.e. 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024 etc