Most images do not have to be a particular size, but there are a few that do. When you start a skin, there will be default images in there that you can use as a guide for size, but for the most part, it is going to be trial and error until you get familiar with it all.
When creating images, save them as .png or .bmp and import them into SKS. If the image you are importing is a .png, SKS will automatically convert it to a .tga.
Read over the tutorials in the wiki and they will get you started. Most of the SKS tutorials are not for SKS7 but the info is still good.
Of course, you are always welcome to ask questions in the forums. There are plenty of helpful people here.