Got an REI Novara at a police auction and have started to tune it up. I noticed that the front wheel is 26 inch and the back is 700mm. The bike rides well (my first bike with a frame small enough to fit good), but I am just unsure of the variance in wheel sizes. I guess the prior owner changed out a wheel at some point and for whatever reason used the different size.

Is having two different wheel sizes a problem? Or should I go out and get a new wheel?

That's 700C, not mm. I doubt the bike came from REI with both those wheels. You should be able to Google the bike model to see if it was a 700c or 26" wheeled bike. Or, go to the REI site, if the bike is recent. Do the brakes work? Are the tires similar? Does the bike work for you? Didn't cost much, did it?

No, it's actually a nominal 700 mm. The "c" is for the rim design specs. A 700 wheel with a 35 mm tire is approximately 700 mm in diameter.

Several years ago a lot of bikes were designed with smaller front wheels. These were popular with the tri-athletes, I think for a perceived aero-dynamic advantage of the smaller front wheel. Terry bicycles has continued to build some women's specific frames with smaller front wheels that allow for a shorter top tube to better fit people with shorter torsos.

Al

Curious to know exactly what model the bike is and exactly what size the wheels and tires are.
700C generally refers to 622mm rims.

622 is the bead seat diameter. The rims are actually approximately 630mm but that varies a little. That leaves 70mm to account for the tires (35mm top, 35mm bottom).

Some bikes had a smaller front tire for smaller sizes to help shorter riders have better control, but I didn't think REI had any models with that feature. I also doubt they had any bikes with that feature for time trials or triathlons either. Smaller front wheels are not allowed in most competitions now.

The bike was likely designed for two different sized wheels and you can't change this easily, if at all. And even if you could, it would change the handling and it might not ride well at all.

You can't put a 700C wheel in a 650C fork. And the brakes won't reach if you put a 650C wheel in the rear. You could perhaps have a custom brake drop bolt made or switch to a rear disc brake to get around this.

I paid $35 for it at auction.

By the looks of it the 700 wheel is stock and someone switched out the front with a 26 inch at some point. The brakes on the front have been adjusted for the smaller wheel. Would be no trouble to refit them to the standard size they were designed for.

That's kind of bass-ackwards by current fashion. Trek makes a "69er" bike with a 700C in the front and a 26" in the back. Theory is that 26" wheels accelerate faster in general, but having the 700C in front helps with rolling over crap.

Probably a bit gimmicky, but there you go.

If the rims and tires are narrow then it sounds like someone got caught up in the Triathalon fever and modified this to make their own tri bike.

Assuming that it was modified then going back to a 700c in front will likely make it feel less nervous to ride. Dropping the front like that quickens up the steering and makes it less self steering.

I don't see how you can do that. I've never seen brakes that can adjust that far. Have you got a picture?