To answer this question, you must know how would like to display your video mosaic.

Please keep in mind that today's popular Full HD resolution is 1080 x 1920 pixels. Most of a modern video players and TV-sets can show Full HD videos without any problems. If you want to export larger video, like a new 4K video resolution, refer a manual for TV-set or video projector that you intending to use.

Mazaika-Video itself has no limits for exporting frame sizes but it is always a good idea to export video frames according to supported resolution.