Hello,
we have this line of code in our atlas system, to find out how big can one atlas bitmap be:

unsigned int maxSize = al_get_display_option(global->display.display, ALLEGRO_MAX_BITMAP_SIZE);

It works on windows, but on mac/unix it returns 0 and we have to hardcode something like this:

maxSize = std::max(unsigned(2048), maxSize);

What might be the problem?

Looking at the source, it should work under Linux. The size is read here:

https://sourceforge.net/p/alleg/allegro/ci/1911416/tree/src/opengl/extensions.c#l877

glGetIntegerv(GL_MAX_TEXTURE_SIZE, s + ALLEGRO_MAX_BITMAP_SIZE);

Do you have a test-case where you read ALLEGRO_MAX_BITMAP_SIZE after al_create_display and it returns 0, under Linux?

As for OSX, yes, it always returns 0 according to the source code. Can you create a ticket for that ^[1]? Someone with a Mac will have to implement it.