I use the quick and dirty method on Windows still with my deskjet printer. Yes there are programs that will divide pics over many sheets for you and you can get any type of interface that you are comfortable working with, but Paint has everything I need. You don't have to be 'exact' either, if it comes out that the squares are 1.1 inches, no one will really notice
I just remember that a sheet is 8.5 x 11. I count the squares used (you may have to check the original maps out of the book since I don't include grids on these) and divide by either 8.5 or just 8 (for margins) or 11, depending on the orientation. That gives me the amount of pages across I'll need. Then repeat for top to bottom. When you set the page setup, take the horizontal and vertical centering off, and choose 'fit to' and put in the page counts that you came up with in the first part of this. Before printing, check the print preview and look at one of the pages that is fully covered. Make sure that you can count 8 to 9 squares for width and 10 to 11 squares in length. If it is too far off, add or delete a page in the 'fit to' section of the page setup. Recheck it until you have the best ratio.
Leave a quarter inch margin, cut the tops and right sides off with a straight edge, use a light paste and it will come out fine.
That is quick and dirty. And even though it sounds a little elementary, the results are way better than you would think.
If you have access to larger printing, just count up the squares each way and that will give you the dimensions in inches you need. I have found that there is usually at least one worker at a Kinkos that has worked with maps and knows what you're trying to do. But really, just playing around a bit before actually printing it, you'll figure it out.
If you have access to a laptop and decent flat monitor (25 inch), I highly recommend setting it flat with a plexiglass cover over it and using Maptool. Download it for free at their site (google will lead you right to it), they have a great forum to learn its functions and everyone will be impressed with the results.
This castle is somewhat of a three dimensional puzzle. For example, all of the rooms on the first floor cannot be accessed easily without going up or down a floor first. Having some form of 'fog of war' that emphasizes how the pcs can lose track of where they are in the castle adds greatly to the feel of the module. Printed maps lose this edge as the players can always see whether they are in the middle of the floor, or at the North end, no matter how much 'covering up' you try to do. And, once you have the monitor ($180.00 for 25"), you stop with the costs of printing. It pays for itself very quickly if you make a lot of maps.
I hope I answered your question well enough and maybe even given you some new ideas to think about.
Happy gaming!