I searched on the forums and a number of people are running 25 (back) and 23 (front). I'm currently running 28 (back) and 23 (front). Is this going to cause any issues? Anything I should know about running this combination?

Michelin Pro3 Race (front)
Continental Contact (back)

should work fine. Just remember, Wider tires require less PSI, but rear tire is holding more weight than front tire, so it needs to support more weight.

Its totally fine.

Perfect, Thanks!

I run a 28 in the front and a 32 in the back...nice combo.
The 32 is a tough Panaracer Ribmo too, not afraid to beat on it.

what exactly is the benefit of riding with a larger rear tire?

Softer ride.

Yep, more comfy ride.
And rolling off of curbs and stuff is more fun.

I think larger, lower-pressure tires resist some types of flats better than do skinny high-pressure tires.

Since rear tires carry more weight and have more flats, it makes sense to put the more flat resistant tire in back.

That said, I ride with 25mm tires, front and back.

I have, formerly, ridden with a Conti Gatorskin in back and a Conti 4 Seasons in front.

Conti doesn't have a Gatorskin on the market right now, since, it appears, that the current 4 Seasons matches the Gatorskin's flat-resistance.

I predict this Fall Conti will introduce a new Gatorskin with increased flat-resistance.

In the meantime, I ride with a 25mm 4 Seasons front and back, and I will switch to Conti's new Top Contact Winter in 28mm as soon as we have our first frost.

Riding with two different sizes of tires changes a bike's geometry, and thus handling, a small amount; but, then, even changing matched sizes of tires changes a bike's geometry, so that matched 23mm, 25mm and 28mm tires all create a different amount of trail at a given rake and head tube angle.

Sheldon says: "Mixing/Matching Tires
Most bikes come with identical tires front and rear. This is all right for general use, but if you want to optimize your bike, you should consider using different tires front and rear. The front and rear tires have different loadings and different requirements.

Narrower Front, Wider Rear
If lightness is the primary goal, tire width/weight is limited by the risk of pinch cut flats, a.k.a. "snake bites." Since there is more weight carried on the rear tire, you can get away with a slightly narrower tire in front than you can in back.

Wider Front, Narrower Rear
A wider front tire makes sense in many applications, however, when handling and ride comfort are considered. A wider tire will generally provide better cornering traction than a narrower one, assuming appropriate inflation pressure.
A wider tire also provides superior shock absorbency. I personally prefer a slightly wider tire in front, since I suffer from some wrist discomfort on occasion."

A limiting factor, or consideration at least, is the tightness of the frame, particularly the width of the chainstays. On my Bianchi roadie, I could probably fit a 28 mm up front, but it's so tight in the back that a 25 might be pushing it.

Fatties fit fine...I had to run something fat in the back.

I run 23c in the front and 28c Randonneurs in the back.

I am running 23 in the front and 28 in the back as well.