For a beginner should i go with a 60cm BJD or a 40 cm?

There is no such thing as a specific doll that is "best" for a beginner or that a beginner "should" get. You should get the doll you want the most. It might take you more time to save up for a bigger doll if that's what you want, but on the other hand if that's the doll you REALLY want you're more likely to have a positive experience with it once it arrives.