This is what you need.... posterazor.
You scan in what you need - tell it how big you want it in inches or by % and it will save everything in pdfs so you can print them out, match up the join spots and bam - you have a resized thing - which is fabulous if you want to make something larger without having to go to Staples and spend more money on it than you want to...
posterazor is free too ;)
http://posterazor.sourceforge.net/