Hi Reigo, well done for taking the leap!
I think it depends on what the sketch is for. If it's just sketching for enjoyment then I tend to use which ever notebook is closest to me.
For sketching at my current work I tend to use A4 as we have plenty of it and I can scan it all in easily, but at Uni and while in previous jobs I used A3 Layout pads.
I don't think it really matters as long as you can communicate the idea effectively.
For work that needs to be presented then I will stick to A3/A4 (one or the other), but I tend to scan these in and edit them on the PC anyway.
It will be interesting to see what others say on this though.
In the mean time, you could post some of your sketches on the doodling board