With Panotour/krpano it merely depends on how much you wish to allow the viewer to zoom into the image, because the multi-resolution tiling (created automatically when bulding a tour with Panotour and Panotour Pro) can handle any size of image.
Shooting hi-res panos is relatively easy (especially using a robotic pano head); stitching, rendering and post-processing hi-res pano images is often quite challenging and demands significant computer power and patience.
Andrew Stephens Nikon D40, Nikkor 10.5mm fisheye, Sigma 8mm f3.5 fisheye, Nikkor 18-55/50/35mm lenses, Nodal Ninja 5 Lite, Nodal Ninja 4 with R-D16, Agno's MrotatorTCS short. Nikon P5100, CP5000, CP995, FC-E8, WC-E63,WC-E68, TC-E2, Kaidan Kiwi 995, Bophoto pano bracket, Agno's MrotatorA. Merlin/Orion robotic pano head + Papywizard on Nokia 770/N800/N810 and Windows 8/XP/2K.
I shoot all outdoor panos with a 50mm equiv lens, as spheres. with 25% overlap that makes 90 images. Panotour makes a tour of ca 200MB to be uploaded to the web-hotel. If there isn't anything interesting to zoom in on I downscale the pano to 71%, then the upload is ca 100MB
What is "optimal size use for spherical photos in virtual tour" ? That depends ! Indoor, small room, church, outdoor, landscape, city ..... I guess there is no single answere to that question.
btw: 5 brackets makes 450 images. then you need power, and it's quite a challenge to get it right !