'How difficult it is to make a
colour print which is actually bigger than the stage size of the RISO SP-275 ?.'
All the work is done when you generate your colour separations - this being on on the computer.
As long as the separations are correct then your screens will be correct.
As you can not image the entire screen in one process, you need to image it in stages.
The artwork is placed against the screen (which has a film side and a mesh side) and you start in one corner of the screen - doesn't matter which one.
Image the first corner - when you do the film melts away and the artwork is lightly stuck to the mesh.
It's this light adhesion - which can be broken by simply peeling the artwork from the mesh - that ensures the remainder of the design can not move around on the screen.
What you have effectively done is 'pin' one corner of the design.
From here you move the screen through the machine to the left (right, up or down, depending upon how the design flows and what corner you started in)
As long as you overlap the imaged section with the unimaged section then the registration continues.
If you image one corner, then skip the middle and image another corner, you have the problem of the design developing a ridge/ roll in the mesh in the middle and the screen is ruined.
Have you seen a sample of the screen with the design attached just after it has been imaged?