I've used the testors and it worked pretty good. Its A5 sized; about 5x8.
When you take your image and resize it (I use Paint) make sure you check the "properties." This will give you an actual dimension of the final product. Much easier than trying to guess and waste decal paper.
Save the image as a BMP. It takes more space, but the image is crisper than a JPEG. Then open Word and set your paper to the A5 setting and set your margins. I make the margins as tight as possible to maximize the number of decals I can have per sheet. Use the insert function to place each image, then either 'tab' or hit the space-bar 3-4 times to put a gap between images so you can cut them out later.
Once your done, print a test sheet on plain paper. THIS IS IMPORTANT!!! The test sheet will show you exactly what your going to get. From clarity to size and color. You can cut out the test patterns and dry fit them to make sure everything fits correct.
Only then do you print out the decal. You get one pass with each sheet...so put as many decals on there as you can. Extras are nice because you WILL tear one or mess up putting it on at some point.
Once the decal is printed I let it sit for about an hour or 2. This lets the ink set and dry (on inkjet printers the ink is laid on top of the decal paper and does not imbed into the paper itself...so it is easy to smear). Once dry I hit the entire sheet with a plain matte varnish. Not to thick, but not thin either. Be careful of temperature and humidity. To hot or humid and the varnish can run or dry before it hits the paper causing rough textures and runied sheets. The varnish seals the ink onto the transfer paper. Let it dry/cure over-night to be safe.
The next day trim each decal to the size you need (I find the closer its trimmed the better they come out). Soak the decal for about 15-20 seconds. Use Micro-Sol and Micro-set solutions (available at the same hobby shop) to set the decal in place. Once dry hit the decal with Micro-Sol again. Repeat at least once or twice more. This dissolves a lot of the film/varnish and leaves you with a really clean decal setting. Once your completely done, seal the model with more varnish to protect the paint/decals.
Kanji decals: