Welcome to the forum Julie, promise I won't swear (too much)!

I have been considering designing a larger machine also and have been lurking around youtube and various 'other' DIY CNC sites for idea's before I put mouse to screen.

Found this site on my travels - http://www.automatedwoodworks.com/

I think this guy has done a half decent job, take a look at his gallery and his youtube video's. He is using steppers which are a much cheaper option than servo's and he is getting decent rapid speeds.

Like other people have said, with steppers its a trade off between speed and accuracy, on my current machine I am using 16 x 5 ballscrews which are directly driven off of a 23 size 3.1Nm stepper, with a 5A driver I can run comfortably at 5m/min and I have had it up to 7m/min. I am running at 1/4 step so this give 800 steps per 5mm, this = 0.0063mm which is fairly accurate!

I am thinking of using the same steppers and drivers for a larger machine but gearing them 2:1, this should double the rapid speeds and half the accuracy but still give me 0.0126mm per micro step. Pretty sure that the 23 steppers will be fine but may consider going up a size to be on the safe side.

Will probably belt drive it and just use one stepper for the long X axis with a shaft to link the drive belts on either side to avoid any crabbing.

Just my two penneth worth!