in ibisPaint X, there will be preset sizes, and you can choose your own canvas size.
the big box that shows the width and height is the one you choose if you want to make your own canvas size.
if your canvas size is more than about 2000 pixels on each side, then it can get a bit laggy :( what I suggest is that you choose a canvas size where the width and length are no more than 1500 pixels. you can resize the canvas when you're done with the edit.
to resize it, select the canvas option in the picture.
then click resize and it will show the size of the canvas. resize it to however you like.
it might take a short while for it to render but be patient :D
that's all for this chapter. I hope this helped to those using ibisPaint X ~