Preface: I haven't designed a microstrip (or whatever) filter yet, myself.
My impression of such filters is like this:
Suppose you want a, say, 6 pole bandpass filter, very narrow. You need 3 L's and 3 C's. The general design of such a filter is a parallel resonator, coupled to another parallel resonator, using a series resonator between (for a Pi design). A very sharp bandpass means the impedance of each resonator must be very different from the transmission line impedance, while the poles are kind of on top of each other (give or take pulling interactions). So the parallel resonators need a very low impedance to successfully shunt the line, while the series resonator needs a very high impedance to keep coupling to a minimum, except in the narrow frequency band where it's desired.
But with microstrip or what have you, it's very difficult to get such a large impedance ratio, so your filter Q (sharpness) is way down and you need more stages instead. This is not done with discrete components, because you can wind an arbitrarily good inductor, and one expensive inductor is better than matching three, smaller, custom inductors.
As I'm sure you're already familiar with, the basic idea of microstrip (or whatever) is to alternate between high and low impedance segments, where the low impedance segments look like low-Z parallel resonators and the high impedance segments look like high-Z series resonators. Or vice versa. Using the impedance of a resonator as the corresponding quantity, it should be very easy to calculate a simple bandpass by trace widths, of course you'd need to model it to verify dimensions are correct and the poles are in the right place.
A lowpass filter doesn't need large impedance ratios (high Q resonators), at least until the higher order poles. Getting a sharp corner could be challenging in that case, but using more stages always works.
You can save on trace width by giving it some height over the ground plane -- you can cut out a hole to give the field some room, but I don't know how to calculate the cutout required. Would also kill EMC.
Tim