how to print an image to a specific physical dimensions on multiple pages

I have a personal project for which i have to print an image and i want it to have a resulting size of 68x68cm. I only have an a4 printer. I know how to print an image on multiple pages, it's easy, but what i need it's a fraction of an a4 page, not a full page.

Let me explain, how i see it in 68cm i can fit 3 x 21cm (a4 height) and i have a remainder of 5cm. Also on height i have 3 x 30 cm it is too much, i need 2 x 30cm + 8cm on the last printed page.

Anyway i hope i was clear enough.

I am a software developer, i have resonable graphics skills (photoshop etc), but i can't seem to find a good solution, except for a custom print of a larger size, which would be quite expensive. Thanks!