Looking at replacing cranks, and different people have told me different things. Is a smaller crank (with a slightly smaller chain) going to make the bike slower? Or behave/feel differently at all? Is this a bad idea (fit with the bike or is larger just better)? Looking to get a little more educated on this before I buy after-market parts to do this. Thanks so much in advance (newbie here)!

Size is not that important. When changing components compatibility and adjustments are important.
Most road bikes and many hybrid bikes have chainrings that are sized for professional racers and too big for most recreational riders. What the pros use sells better.

Al

Behave/feel differently from what?

And what do you mean "smaller"? Different chain wheel sizes? Yes, changing the gearing is something you will notice.

Feel differently from smaller chain wheels - exactly. Is it just a feel thing though or will the bike perform differently (speed, etc)?

Thanks!

Yes it will perform differently.

"...Is a smaller crank (with a slightly smaller chain)...
It depends on just what you mean by smaller crank.
Smaller rings are going gear you down just like if you put larger cogs in the back. If you are already "spinning out" in high gear, a smaller ring will exacerbate the problem.
Smaller (shorter) crank ARMS will give you a different effect, which may or may not be good, depending on how well your legs match your current cranks.
With shorter arms, you lose "torque", but tend to be able to spin at a faster cadence.
If you have arthritic knees, shorter may be better since you don't have as large of a range of motion (less bending) of the knees. If you have very long legs, possibly a longer arm is better for you.

Size is very important. At least my wife says so.

The motor is what makes a bike faster or slower. If you put a Ferrari transmission into a Yugo the result isn't going to be a Yugo that cruises at 150MPH. If you want to cruise at 150 MPH, you have to have a motor that makes a lot of horsepower.

If you put smaller front chainrings or 650c wheels onto a bicycle, unless the rider is so powerful that they are constantly spinning out in the fastest gear combination, the speed won't be affected.

And her basis for comparison is....?