No idea about aftermarket decals, but resizing and reprinting yours might be do-able.
Making your own decals is a relatively straightforward (and surprisingly easy) process, with a reasonably small learning-curve. You could make a high-resolution scan of your smaller decal sheet...or make up your own artwork from online images and lettering programs, even in a graphics program as simple as MS Paint. (MS Paint has many limitations, but it's also simple and fairly intuitive to use---I've made lots of decals at home, and 'Paint' is what I've used for about 95% of them.)
I then typically resize my finished artwork in 'Open Office' (again, because it's easy to use), and print out on my trusty HP printer on either white or clear inkjet stock, whichever is suited to the project at hand. Once fully dry, I use Testors Decal Bonder to fix...it seems to cause almost zero runs and 'blushes,' compared to other stuff I've tried. Once the bonder is dry, the decal can be trimmed and applied as usual. (Note that images printed on white paper must be trimmed close---unless going on a white background, of course---or a matching 'border' of the intended background color actually printed as part of the decal image. Even then, a little 'touch-up' to hide the cut white edge underneath may be required.)
[The usual 'your mileage may vary' disclaimers also apply. Different printers' inks and decal paper combinations may require a learning curve or 'dial-in' to get right...and some printers may refuse to cooperate entirely. But my personal experience has been that, once you find the 'setup' that works for your equipment, decal-printing can be pretty easy and routine.]
That's the basics. Hope it helps!