Tire Change Compatibility I'm trying road bikes for the first time, and I'm rather unfamiliar with tires and how things are intercompatible.
I'm thinking of getting a cheap Merax 608XC, and plan on taking it on trails such as the C&amp;O Canal towpath. I've heard of cyclocross and touring bikes, but simply don't have the budget to afford any. I'm currently planning to get the merax and swap out the tires, but I don't really know if switching out the 700x23c tire for a 700x28c with knobs is possible?
Save your money and get a used bike instead (look for bike shops in your area that sell used bikes, or craigslist or something). 230 dollars should net you something decent from the 80s or 90s, along with some accessories (helmet, locks, lights, etc.). It'll be much higher quality than what you're looking at (the bike you've linked is bso-ish), and older road bikes have much larger tire clearance normally than newer bikes.

One of the big problems with buying your first bike online is sizing -- not everyone fits the sizing charts bike manufacturers may publish. And you don't get a test ride online to know if it will fit you well. 

The C&amp;O canal towpath just has a bit of gravel on it, not much terrain toughness. I'd not worry about knobs, just 28c+ tires. As for sizing, you have to try the tires with the particular bike, especially with a road bike. Normally with modern road bikes, you can maybe go 1 tire size up with a smooth tire compared to whats originally spec'd. You've gotta check the tire clearance to see if the tire will fit (also note that two new tires will run 40+ dollars on the low end). So, I'd conjecture that the 28c tire won't fit.