I'm working on my first city, and have been unhappy with the scale that I have used for the size of the city, and was looking for some guidelines or tips.

The actual city is Corbhen, The City of Mists that is situated in the kingdom of Ord, for those of you who are familiar with the Iron Kingdoms setting. Its a fantasy / steampunk city of originally 100,000 people, but is now down to approx 50,000. The city is described as being walled with a few sizable buildings that are used as landmarks (Castle, Cathedral and Coliseum). Its an old city that has been built, conquered, almost destroyed and rebuilt more than once. Technology level is set at about the early Renaissance.

So my questions are something along the lines of the following ...
What would be an "appropriate" size for a city of 100,000? 5 Square km? 10 Square km's? Is there a "rule of thumb" that you use?

Thanks in advance for the help.