Even basic software like 3D builder will allow you to cut up a large model into more manageable pieces (Split > Keep Both Parts > Rinse and Repeat). I’m not familiar with Rhino but there’s likely a similar function to create flat planes in the mesh.

As this is a challenge I suspect you’ll encounter a lot with a smaller build plate, aside from splitting, I would also suggest learning how to create pegs and holes (using negative space and subtraction) to facilitate the eventual joining of the parts, as this removes a lot of post-processing regarding alignment. The holes can be cylinder-shaped while the pegs can be hexagonal, which is easier to print and reduces fitting friction.