What size should account avatar images be

I have tried to set an avatar image use PNG and JPEG files, but it was cropped by emClient. I resized the image files in Paint3D (halved them for starters) and I get the same thing. The image is cropped, but cropped at the same point as before, i.e. it is the same avatar image as before even though the original file is smaller.

How do you size a PNG/JPEG file so it all fits in the Avatar ‘window’?

If you use a square image, so the same vertical and horizontal pixels, it will fit exactly within the borders.
Otherwise you can zoom in and drag the image around to choose the best fit.

Images that we source from Gravatar are 50x50 pixels, and they appear quite usable in the eM Client interface, so there is not really any need for them to be bigger than that. Some providers also limit the size of the image you can use, so bear that in mind.