From my vague memory, the ITU used to limit phone numbers to a total of 16 digits (that includes country code). They recently (within the last 3 years) had to increase that limit and I think it is now 20 total digits which means that there aren't any phone numbers that long yet. So you'd probably want to allow something like 32 character so you've got room for formatting. Having to decide field lengths is one of the things I hate about most relation databases.

I'd recommend storing the country code with the phone number myself.