Hi Liz,
Lead Source has a field type of string which means that it can contain up to 255 characters as you define.
You can put multiple values in the field seperated by a comma or semi-colon. However, this will change how the Lead Source filter works. For example, if you have a Lead Source of "web; form fill" you will not be able to catch this lead with the filter "Lead Source is web". You will have to use "Lead Source contains web" as it no longer matches up exactly.
Does that make sense?
Will