It's a fairly complex subject with many factors, but to understand what you want to do, it helps to know why it is the way that it is. It seems that Mercedes wanted the smart car to have extremely safe handling behavior, and whether that was due to a rumored early failure at a European emergency swerve test, or due to some internal company decision, really doesn't change the outcome that the 451 has extreme understeer built into the handling behavior. This heavy understeer means that traction is lost at the front wheels first, that in steady state cornering, if you increase the speed you will require more steering input to maintain the same turning arc. That was accomplished in four ways.
First, as you've noted, the front wheels are smaller and narrower than the rear.
Second, the front wheels are closer to each other (this is called the track) than the rear wheels, by four inches. This creates a turn vector that tends to grind the front outside tire by increasing the slip angle at the front wheel.
Third, the alignment specifications that call for a lot of front toe-in with minimal rear-toe in, combined with minimal front negative camber combined with moderate negative rear camber. These specifications increase grip at the rear tires and minimize grip at the front.
Fourth, there is a front anti-roll bar, but no rear anti-roll bar. The spring rates don't compensate for the lack of a rear bar.
Changes you can easily make can increase the front traction but will only moderately affect the understeer attitude. Even if you were to use front tires twice the size of the rears, the car will still require increased steering input as you increase your speed in a steady state corner - this inherent handling trait can't be changed without doing something relatively expensive such as changing springs or adding a rear anti-roll bar.
You primarily want more comfort, and that usually means tires with a taller sidewall. That also helps because the speedometer tends to be about 6-7% optimistic, so that an indicated 60 MPH is only about 56 MPH. (The odometer, however, is spot on with the stock tires). Taller tires that can fit the stock front rims are hard to come by, and using the wider rear rim on the front helps to get the front and rear track a bit closer to each other. That's why one of the most common upgrades is to use four rear rims.
By doing that, you can run a taller tire since more sizes fit the rear rim width. Many people run the same sized tires at all four positions and that's a good improvement. You can then rotate tires as well, which will give you a bit more tire life overall. The traction and handling assist systems also won't mind that all four tires are the same size. The stock stagger has the front wheels being a bit more than 1% smaller in circumference than the rears, and the stability system seems to be okay with a range of 0% to around 2.5% smaller up front.
The other factor is that you have a slight rearward weight bias, so a slightly larger tire on the rear would usually be typical. On a low powered vehicle like a smart, with the inherent understeer built in, it's not a big deal and it's fine to go either way.
So with all that being said, for what you initially said about wanting comfort, you are probably best off by getting four similar sized rims of 5" or 5.5" width, and going with 175/55-15, 185/55-15, or 185/60-15, depending on how much cush you want in your ride opposed by how much acceleration you're willing to sacrifice (as the bigger tires will effectively increase your gearing and require more power to spin up to speed).
To reduce some of the understeering behavior, you can put some spacers behind just the front wheels. This will help equalize the track width of the front and rear. Also, getting an alignment that sets toe-in at the front wheels to around 1/16" or around 0.05 degrees, helps quite a bit to reduce sidewind sensitivity and minimizes tramlining on grooved pavement.
