about 8x10" is great to hold in hand, portfolio, samples etc and some may look good on the wall too.
normally, bigger than 20x24" is challenging to do in the darkroom. rolls are needed instead of sheets, trays are not big, and those for 20x24" are already not easy to handle, especially if done alone.
but in the right mood (experimentalism, passion, vision, need) everything can work - different materials, odd sizes etc. the endeavor itself is cool, and the results are exciting many times.
size and printing technique depends on the content and the desired aesthetic rendition. theres something "photographic" in the darkroom crafted print that is big enough to engage, but not too big to loose the contact with the frame. so besides the experimental vibe, the 8x10 to 20x24 sizes work very well actually.
presentation, lighting, and other viewing aspects are important too for a complete impact of the print.