edcculus summarized things well. You need to be able to handle the sensitive materials you use without fogging them. The safelight should also not be so bright as to interfere with other darkroom operations, like focusing the enlarger. In any case, periodic tests are advisable. Size doesn't mean much. My safelight is based on a two tube 48 inch shop light, and it provides about the same illumination as a properly fitted Wratten No. 2 dark red lamp assembly. The Thomas safelight is based on a low pressure sodium arc lamp. While quite small, and quite safe, it gives out so much light that it is like working in a well lit room, and you have to shield the enlarger to focus.