Yup, that’s my usual fee as well.
b.easton at exemail.com.au
For the hex grids, you need to create a separate hex grid for each (newly cut) board. You can usually just copy the existing grid from original map to each board, if the artist was accurate, that should still be the right size and you just slide it around to match.
Then you need to adjust the start numbers for the vertical/horizontal grid numbers to match the starting hex on each board. It’s confusing, because the grid numbering calculates itself based on the whole map, not the board and depending on the reverse options, you may need to enter quite large positive or negative numbers as the start numbers to get it to work. Keep playing until you get the reported grid numbers to match the printed grid numbers. You need to do this on each board.
The usual problem people have is that they try and cut the map through the hex centers. This nearly always ends badly.
Assuming the flat side of hex is horizontal, make the horizontal cut halfway between the hex centre and the hex edge. Make the vertical cut at the left (or right) end of the horizontal hex edge. This keeps the hex centers away from the split between boards and keeps the snapping and stacking working cleanly. The new map pieces do not need to be the same size. but obviously have to make up the complete map.
Apart from that, any good quality image processing program should allow you to drag out some visual guides, and then cut and paste each map section into a new image. With a map this large, it can be difficult to this cleanly by manual click and drag unless you are very careful and zoom in afterwards to check that your selection actually matches the guides. In the GIMP, you can zoom and manually adjust the selection after the drag select.
Alternatively, use the Canvas Size or equivalent dialog to cut out pieces of the correct size from the original map.
I usually use an individual part-map size of around 5000 x 3000, but that’s just by chance due to the hex size I use and the size of the individual maps in the project I was working on. For you, I would start with 4 maps 10Kx8K (ish) and see how that loads before doing any grid work. You might get away with that, but that is still on the large size. The more cuts, the more grid fiddling you have to do. Next step for me would be 8 maps 5K x 8K which I would expect to be fine.
What is you standard hex size out of interest? I use a 110 pixel flat to flat size with 75 pixel counters. Any bigger than that and I would consider scaling your artwork smaller.
Anyway, send the link and I am happy to have a look.