so for my birthdaycard i tried to work with a bigger canvas as an experiment

3252 x 2480px at 300dpi may not be that big (its DIN-A-4 size when printed out) but you definitely feel the difference when doing stuff like gaussian blur or bevelling because it takes a significant time longer to apply

filesize is 45 MB, but in Gimp its 1,2 GB

this leads me to the question: how do you work on stuff thats even bigger, like posters ?
is that even possible ?

I tested the size you specified (which I'm using sometimes too). Made several color layers with various layer modes, made some Gaussian blurs. Memory consumpition between 300 and 400 megs. Linux, i5 750 processor, 4GB physical memory. No serius lag. Gaussian blur took some seconds.

Maybe masking and some other things would have slowed it.

I guess it would work OK with A3 300dpi, but... Well, I don't much need that big. I guess if really big is needed, more memory and heftier CPU should handle it. Some lag is always expected.

Honestly, current machines are really easy to work with. I remember trying to paint somethinh big with 80486. That was... slow.

It is, and it takes forever to get anything done, but the level of detail in the final image is a lot better. Text is crisper, blurs are softer, that sort of thing. It gives you, in the long run, more options and the ability to scale it down to the size you need for your final work.

The biggest image I've worked on was 8100 by 4100px and it has a print size of 2.8 metres by 1.4 metres. It was on my old PC, which was quite a lot less powerful than the one I now use, but Gimp seemed to handle it quite well. The image was a map of Mars that consisted of lots of small sections that had to be aligned to make up the final map.

I usually work with canvas sizes on the order of ~6000px. Changing the tile size in settings helped a lot with speed. I have 12GB RAM installed, and it'll all eventually get eaten up.

MyPaint seems a lot faster and leaner, and that's one of the reasons I started using it for the majority of the work that goes into a drawing.