Hello there. This is something I often deal with when printing my maps. everyone is right that you should use posterazor. Here is the step-by-step:

--Load the image. Click next.
--Set this to "custom" (trust me) and choose the actual size of the "tile" you want on each page. If you are printing on regular page size, I'd recommend doing width of 7 and height of 10. Set all 'borders' to zero. Click next.
--Set "overlapping" to either zero or 1 inch, depending on how much 'wiggle room' you want when cutting it out and placing it together. I always do 'zero' to save ink and am just careful when I cut it out. Click next.
--Choose "absolute size" and set it to the number of squares in width and height. Set the image alignment to center. Click next.
--Save the PDF.

Now, when you want to print it, open the PDF. Click print. Set "Page Scaling" to NONE. Uncheck the "choose paper source by PDF file size." Click okay.

This should give you precisely sized 7x10 tiles with a little border you need to cut off. Slice 'em up, tape them together, and you've got a perfectly scaled map.