The way I would approach this is to try and turn it into a backdrop border. The attached image is the border for a chat bubble. In this case, the edgeSize property of the backdrop would be set to 32, so the image is divided up into 8 32x32 pixel squares (as marked). You will need bigger squares, looking at the sizes involved. From left to right the squares are for: left border, right border, top border, bottom border (these two appear to be rotated), top left corner, top right corner, bottom left corner, and bottom right corner.
What I would do, (although I've never done this before), is turn your image into a border like this one. You'd probably want 128x128 pixel squares, and you might have to tweak some distances slightly, but then you could make your frame any size you like, and have the border still work fine. It looks like your background is solid color, so you might not even need an image for that.
Finally, you could use it like any other backdrop, for example:
lua Code:
frame:SetBackdrop({
bgFile = [[Interface\AddOns\MyStarTrekAddOn\frameBg]],
edgeFile = [[Interface\AddOns\MyStarTrekAddOn\frameBorder]],
edgeSize = 128,
insets = { left = 128, right = 128, top = 128, bottom = 128 }
}
frame:SetBackdropColor(1, 1, 1, 1)
frame:SetBackdropBorderColor(1, 1, 1, 1) -- if you like, you could even make the image monochrome, then tint it how you like.
As I say, never done this before, so might need a bit of tweaking. If you get it working, please post to say how you did it
.
Hope this helps,