First, welcome om board. Is it a wall mount type like mameroom.com you seek for? I know they have plans to sell, but you of course not need to any buttons as well... Otherwise I have not created a jukebox cabinet by myself, due the tiny place I have.
About the touchscreen size I guess it depend of the jukebox app chosen. For a wall box I think about 14" -> 17" would been very fine as long it have around 800-600 to 1024x768 resolutions (or above), due most software use that resolution.
Otherwise it for the PC hardware, this depend very much the song collection you have (singles, albums or both), how have set it up (example have you tagged them, or use a file mask system), and if you have videos as well.
For most chase a 1ghz processor around 1gb ram should been fine for most software out here. Some can even run on less hardware on these. On the graphicscard I think it only my own software that might require a little bit more than others, since it use OpenGL and not GDI as most others due). But for the soundcard part, any soundcard can been used, but of course better soundcard have more clean sound and procures less noise.
With videos it might require a bit faster processor......