Welcome to Seven Forums steamer. Looks like you can edit the .js file and change the maximum size settings (320x240).
Navigate to C:\program files (x86)\Windows Sidebar\Gadgets\SlideShow.Gadget\en-US\js folder, and inside you will find slideShow.js> Right click slideShow.js and choose Open with> If Notepad is not a choice listed, click the small arrow next to Other programs and choose Notepad (other programs can work, but notepad is easiest)> Scroll down to
var maxUndockedWidth = 320;
var maxUndockedHeight = 240;
Here you can change the maximum sizes. You may have to keep the aspect ratio the same (4:3), but you might experiment with 16:9
Looks like you would have to change the BG and frame numbers as well to match the other changes. You'd have to make all the numbers the same and make the correct width and height adjustments for the other settings. IE: if you change the 320x240 to 640x480 (double size, easier to do the math), the other settings (360x280) would have to change to 720x560
gUndockFlag = true;
with(document.body.style)
width=360,
height=280;
with(slideshowBg.style)
width=360,
height=280;
slideshowBg.src="url(images/on_desktop/slideshow_glass_frame.png)";
with(bar.style)
width=112,
height=23,
top=220,
left=119;
with(pictureFrame.style)
top=16,left=17,width=320, height=240;
Click the Close X upper right and choose to save changes> See how it looks
There may me a limit to how big, and the display quality may suffer at larger sizes. I would make a copy of the original slideShow.js file to restore just in case it gets screwed up. Good luck, let us know how it went. A Guy