The size of the sig you request, or pick, is usually based on whatever your preference is. A sig the majority of the time is an even height and width (400 x 100) not (401 x 99).
What does 400 x 100 actually mean?
The first number (400) is the width of the image in pixels. The second number (100) is the height of the image in pixels.
A few factors come into play when choosing a sig size:
1) Where do you plan on using it?
If the majority of the time you plan on using it on a certain forum make sure that forum doesn't have a physical or file size limit. You'll often see "sigs can be no larger then 500 x 200" or "sigs can't exceed 400kb in size". This is to keep the pages/images loading quickly, especially if the forum is hosting your sig (if you upload your sig to their forum via a control panel then that is what is loading it each time).
2) What is your preference?
Smaller sigs mean that there is less room to work with design wise, so certain graphics might be too small to be visible. Sigs that are really tall though can look odd and most forums restrict the height so the scrolling between each post is decreased. You most likely want to pick a common size or a size that is used often on the forums it will be placed on. If sigs are too wide it can throw the entire forum out of whack spanning it further then it needs to be, so again most forums will restrict the width.
A safe rule of thumb is pick a standard width(somewhere between 400-500 pixels) and height(100-200pixels), check out the sizes of other sigs on the site you use and you should be set.
Here are some example sizes:
I hope that gives you a good start on choosing a sig size!