Now the long version.
Obviously, you want a map tile which is a fair size so you should use A3 as your starting point. However, few printers can print right to the edge of the page so you won't be able to use the entire area. Also, remember that we need a one inch grid for D&D battles so we need to round the size down to the nearest inch.
I used to create map tiles that were as big as I could fit on A3 with the printer I had access to - 16 inches by 11 inches - but later moved to square map tiles so they could be arranged in more versatile ways. A square map tile on A3 can be up to 11 inches but that was so close to 10x10 that I thought I might as well go for a round number. It makes working out ranges simpler if each map tile is 50ft (assuming a standard five foot grid).
Making a 20" x 20" canvass in Photoshop means four 10" map tiles in a 2 x 2 grid. That's enough room to draw some decent sized battlefields without worrying about the map edges but how many tiles you want probably depends on the size of your gaming table more than anything else. Most are longer in one dimension than the other, so maybe a 2 x 3 map tile grid (or 20" x 30") would be better. Whatever the size, you can later chop them up into 10" x 10" tiles for printing.
Then there's the issue of resolution. These maps are going to have a large number of layers with complex filters and layer effects on them, so you can get a fairly hefty files size very quickly. If you have a powerful computer with lots of memory, Photoshop should be able to handle it, but a lesser computer might find it easier if you reduce the resolution of the image. In practice, I found that people wouldn't notice maps even as low as 100dpi. 200dpi is probably safe and 300dpi should only be used if your computer has the horsepower to handle it. However, note that you can always reduce the DPI later but increasing it is complicated enough that you might as well not bother. With that in mind, I choose 300dpi.
Finally, as we are going to print this out, it's best if the colour mode is CMYK, which is what printers use.
1.2 Create a Grid
Not everyone plays D&D with a grid but even if you don't print it out, a grid layer is a useful reference when creating a map. Photoshop makes this pretty easy.
- Create a new image which is one inch square at the same DPI as your main map. So, for me, this is a 1 inch by 1 inch by 300 dpi image.
- Create a new layer.
- Go to the Edit menu and select stroke. Create a 2 pixel, black inside stroke. (You can make it thicker if you wish but two pixels seems adequate.)