I'm sure there are others out there, but this one works for me. It allows you to test your expression with variations on your data to make sure you getting exactly the results you want, and displays diagnostic information on how it got those results.
You didn't give a lot of information on how your data can vary, but my first instinct would be to make it as fuzzy as possible without failing. Your regex works as long as there is always one single digit, a period, and one single digit. I assume this is a storage size or something similar. You mentioned a max size, but can it be less? Will it always display a single decimal place? If its less than 1.0 will it display the leading zero? Is it always provided as "G" (gigabytes?)? And so on.