If the bike is comfortable and you can stand over it, I wouldn't worry about the size. Bike geometry is always a series of trade-offs, especially in the smaller sizes where the angles get manipulated to make room for the wheels.
If you have some money you'd like to put into upgrading the bike, I'd start with the tires. Nothing changes the feel of a bike more than nice tires. Also, commuting and CX racing require very different tires (unless you have an off-road route to work).
After that, I'd go for pedals and saddle (though only if you aren't happy with what you have).