Blowing-up a file

[Guy Glorieux <guy.glorieux@xxxxxxxxxxxx>]

I am interested in knowing the group's experience with blowing-up files for 
printing to very-large format.  I am working with a Sony A100 10 megapixel 
DSLR and I am curious to find out how far I can blow-up a picture.

What is the best program to use?  How far can I go on enlarging the file? 
What is the best procedure to use (i-e a 1-shot increase with unsharping at 
the end or several smaller size enlargements with a small dose of 
unsharping).

Thanks in advance.

Guy