Do SMT component leads have to be the same width as the transmission line?

not really since it is a low % or wavelength

but what would be the optimum width?

optimum for what?

No. There is a discontinuity at the component anyway, because the current flow inside the component is not at the "bottom" of the component, and the component dielectric material is different from erel=1 above your microstrip line.

In many cases, having a physically small component (small in width & length) reduces the parasitics and results in better RF performance.

In other words as an example, if you interrupt a straight microstrip line with a 0 ohm resistor having the same width of the line, the performances of the microstrip line (as loss, impedance, and phase) will change, due to reasons mentioned above.
Of course, this influence can be seen better at high microwave frequencies.

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