Hi Jan
It maybe that you selected an 80 x 80 Avatar originally as that was the largest you could have, in order for your picture to be larger you could try selecting one that is sized 120 x 120
If you are not able to find the Avatar you already have in that size, you could make it larger but may lose it's sharpness, or you could choose yourself a nice new one, maybe an animated one! - just be careful that the Avatar you choose does not exceed dimensions 120 x 120 and file size 34.2kb as that is the maximum you can have.
To find out these details before you choose your new Avatar, when you find a pikkie you like, right click on it and select 'properties' it should tell you file size and dimensions
Jan, as an example, if you right click on my Avatar, you will hopefully see what I mean about the dimensions and size etc.
Hope you get on okay, if you need anymore help let me know!