This is absolutely why you are having to call the constructor...just tried these changes here, and it works...but I guess Christian has already seen it. You should also note that to really do the inheritance properly (in c++ish fashion), you'll should update the constructors of all the sub-classes to take the width and height parameters. Something like this worked for me:

// Constructor for 16x32 panel:
RGBmatrixPanel::RGBmatrixPanel(
uint8_t a, uint8_t b, uint8_t c,
uint8_t sclk, uint8_t latch, uint8_t oe, boolean dbuf):Adafruit_GFX(32,16) {

init(8, a, b, c, sclk, latch, oe, dbuf);
}

But really, to be more "generalized" you should pass in the W/H values in the sub-class's constructor: something like:

// Constructor for 16x32 panel:
RGBmatrixPanel::RGBmatrixPanel(
uint16_t w, uint16_t h,
uint8_t a, uint8_t b, uint8_t c,
uint8_t sclk, uint8_t latch, uint8_t oe, boolean dbuf):Adafruit_GFX(w,h) {

init(8, a, b, c, sclk, latch, oe, dbuf);
}
And of course remove the explicit constructor call from the init function in RGBMatrixPanel. I imagine all the sub-classes would all need to be updated similarly. Anyway, just my 2 cents...hope it helps :)

This has been manually rolled into the latest update. Really appreciate your help with this one -- thanks!