Ship Building basics.

Hopefully, this post is in the right section... To the master ship builders we have for the game; How do you know if your custom ship is the "right" size for the template you make?   

For example, I would like to make very similar designs from tiny, small to medium...  When resizing of parts or creating from scratch I am not sure at times if my tiny craft becomes too large or same for small or medium.  I know the basics but am not sure if I spend hours on a tiny unit and then in game it appears more small size.  

I am trying to get better at ship designs and hope there would be a source for tips on ship building. I really enjoy this part of the game. I design MANY Civs but would love be proficient on ship building.