So, how big is yours?

First, get your dirty minds out of the gutter! :p

Well, I started up some more ships stats after a few months off. And I always find myself wondering how much mass the ship should have.

Anyone have any suggestions on figuring out how much a craft should weigh?

I have been considering using the “SU” and “Mass” ranges in the Construction Manual. For example: a size 6 ship has an SU range of 1,300 to 2,500. If said ship in question used 1,600 SU’s (or 25% of the SU range) said ship would weigh 25% of the mass range (300,000 to 1,750,000) or 662,500 metric tons. Another example being a size 6 craft using 80% of the SU range would use 2,260 SU and weigh 1,460,000 metric tons.

Anyone using something else, have an idea, or see a way to improve my idea?

That's the same formula I use. There are exceptions however;
1) Singularity Core - Double the mass of the core and nacelles.
2) Ablative Armor - Double the mass of the armor.

The two devices, IMHO, have a much greater density than other systems listed, on average.