I've got the regular size and @2x for iOS devices. But what is needed to cover all the Android devices? I've added @1.5 which works for some but still missing something.

Android devices are:

Phones:
320x480
480x800
480x854

Tablets:
1024x600
1280x800

Personally I use the following:
320x480 (for iphone/itouch)
480x800 (for android phones, and stretch by 54 pixels for droids at 480x854)
1024x768 (for iPad, and downscale for android tablets at 1024x600)

I recently added xoom support which used 1280x800. I upscale some of the 1024x768 graphics as there's not a massive difference.

You can use some of the tricks (section 3) on my page to handle the images:

http://www.base2solutions.com/walkabout/Corona%20Tips.html

Use letterbox and corona to size up the graphics but override stretch/zoom for odd shape devices like the 480x854 droid.

All the regular graphics are working fine. My problem is from the sprites on the Galaxy Tab simulator. It's using the @1.5x size sprites but they're coming in too small.

I'm using the crawlspacelibrary which has this:

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scale, suffix = display.contentScaleX, ""
if scale < 1 then if scale > .5 then suffix = "@1.5x" else suffix = "@2x" end end