If what you want is to be able to use designs that you already own, my best advice is to look into this program.
http://www.wilcom.com.au/PRODUCTS/TrueSizer.aspx
It is a free download. I installed it into my computer years ago and it worked great for re-sizing.
There are a few things to remember. I am not an expert digitizer, but have learned some by trial and error. When you re-size, you are not increasing the number of stitches used to draw the design. You are just making your stitches a little longer for the final design to be bigger. therefore, if you take a small design and make it bigger, you will have more area to cover with the same number of stitches and you may have empty spots or a lees filled look.
In your case, you want a bigger design and make it smaller, your final piece may look fuller and for some designs that is great. I suggest making a practice piece before stitching it on anything expensive. You may waste some thread, but at least until you are familiar with the program, is worth it.
This program is also able to convert digitized images from one format into another. Ex....hus to .pes. This is great if you find a design you like but is not available on your format. What I found is that sometimes a color may be missed and there is nothing you can do about that. If you wish to do that, do it only on simple designs, nothing too complicated.
I purchased a design that was 7 x 7 when my machine only take 5 x 7 and was able to reduce it enough to be used with no lost stitches.
Good luck.