Is there much information out there on system ships (non warp capable) in the source material.

I assume one could design a system ship much like a normal void ship but not install a warp drive or gellar fields. But I am interested in any material, stats or simply narative, on the dedicated system ships.

How large should they be? My assumption regarding the massive ships of the 40K universe is that there is a minimum size needed to pack a warp drive, sos sytem ships may be considerably smaller in size.

The booklet that comes with the GM screen includes stats on basic system ships. They are 0.65km long I believe. Also, Damaris has several system defence ships described. As a GM I tend to lean towards a wide variety of sizes depending on usage and distance they need to travel. Remember that it takes weeks to get from one zone of a star to another. That means that any ship capable of traveling between planets needs to have fuel and life support for weeks or months.