Is there any reason why not?
If its your first CNC. Don’t go further than default scale. If You get more experience building your own designs, go ahead and do it. Bigger machine = less stiffness
Most of my work take place between 50cm x 70cm. It’s all calligraphy.
Yes torque will be increased by over 50%, the single row of v bearings will still work, but you will probabaly have to cut 50% less deep and it already is not that fast of a machine. Certain hard materials like aluminum would be even more difficult to do.