Customized Text Window graphics

[Aquilo]

What is the maximum/optimal size (width) for an upper/lower side sprite in a customized text window? I have tried different sizes, and it seems to be a limit when the width exceeds 32 pixels: a tiled upper/lower border often sticks off the right edge of the text window (the tile begins on the left side or am I mistaken?) The ClipImage property in the GUI editor doesn't fix the problem. Is there some technical limit for such sprites or the only solution is to shrink the width? Screen size is 640x480.