I've just tested it out and I can resize the canvas to 500px both before and after importing an image without problems. Could you provide the steps to reproduce your issue?
The 'frame width' and 'frame height' sliders in the import window only go up to 128. You can import larger images by right clicking on the slider and typing in the size. Took me a while to figure it out... I was about to apply for a refund on Steam and then I worked it out.