I finished up my blog series on designing realistic spacecraft for games. I ended up with ten guidlines for rocket design, and I though I share them on the forum. If you want to read the full articles they are on my blog.
RocketDad's Ten Rules for Plausible Spacecraft Design:

1. What's its function? No, no, not its "real-world" function, its function in a game. This is as important as defining the mission is to an engineer, as the in-game function will dictate weapons armor, speed, size...uh, everything, really
2. An RPG Spacecraft Must Look Cool. If you wanna sell a ship - either to your players or on the open market, it must look cool. From all sides.
3. The Interior is as Important as the Exterior. It should be consistant and visually appealing. Make sure to include lots of elements that make it interesting to run combats in, and don't forget the treasure!
4. Unless they're Power Gamers, Players would rather have an interesting ship than a powerful one. The Firefly-class is one of the most popular ships out there right now, and its unarmed. BTW, If your players are Power Gamers...make 'em play Paranoia. That should take some of the starch out of their trousers...
5. The more powerful the weapon, the more limited the rate of fire/number of shots/ targeting accuracy/ something else that keeps it from breaking the game. No one wants their Player running around the galaxy in a Star Destroyer. The Mellineum Falcon is almost as bad.
6. Fictional Spacecraft are Either Anthropomorphic or Iconographic.
This is part and parcel with the human condition. Even real spacecraft, whose designs are influenced far more by money, politics and the inescapable physical limitations of chemical, disposable rockets, fall into one of these two categories. Look at the Soyuz, its practical, yet instantly recognizable as Estern Bloc Space Chic.
7. There Are No 10'x10' Rooms in Space. Observe the case of the International Space Station. The ISS is the single most expensive construction project in human history. This includes things like the Manhattan Project, the Great Wall of China, and the estimated cost of the Great Pyramids at Giza. The total cost of the ISS is greater than the Gross Domestic Product of some industrialized nations.
I mention all this so that you will understand my point about extra space in space: The habitable volume of the ISS is roughly that of two 18 wheeler trailers. There simply is no spare room in spacecraft. All realistic spacecraft are designed for minimum weight and volume. Accept this and go on.
8. Keep the Design Grounded in Reality.
Science fiction is, by its very nature, fantastic. It takes a good bit of willing suspension of disbelief to put over the stuff that is actually true about space travel. The stuff we hope to do later on, like interplanetary travel in weeks instead of months and terraforming Mars, is simply beyond the pale.
9: Does the Spacecraft in Question Have a Point?
"It looks cool" is not a good answer. Take a gander at the sub guidelines for most Star Trek blueprint websites. You will see some variation of the following, "No Uber Dreadnoughts, Please." This goes double for hard sf designs.
10: Make Your Ships Unique.
By this I mean, "Do not design a dozen small orbiters when one or two will do." Making other types of spacecraft Keeps your designs fresh and exciting.
Plz leave a comment if you agree or disagree with any of the above. If you want clarification on any of points, please read the blog before posting your question.
Hope you enjoy!