The "Page not found" message sounds like a bug, and you might want to report it as such on the top right menu of the screen.
I think that the reason that your animation appears to be getting smaller is that you are resizing the canvas, but not the image itself. So, for example, if an image of a frog is 100px high, then you make the image 500px, the frog part is still 100px high, but the entire image is bigger, so there's just more space in the image around the frog. The reason it would look smaller is that when the entire image gets bigger, the animation tab will zoom out so you can see the whole thing. Then the part that is just the frog looks small (taking up 1/5 of the workspace instead of the whole workspace).
To fix this, first crop the image around the part that you want using the crop tool, using the bottom tool in the menu at the right of the image. Then, when you resize the image to make it bigger, make sure that you have clicked "Resize canvas content" so that everything inside the image is also resized.