Sav growth

What is the average size of a 6 month old savannah monitor?

This is really tough to factor just because it's 100% based on care. There really isn't a set average.
I grow mine pretty big and males seem to put on some serious size quickly. I had two males hit over 2 foot by 10 months old.
Females seem to be smaller in that they would hit about 20-22 inches by a year old but potentially larger.
You can factor that a properly cared for animal will hit about a foot long or more by 4 to 5 months old. That's my average regardless of what the sex turned out to be. Most species of varanid put on the most growth in the first two years of life when properly cared for.
If you didn't raise it though there are no guarantees or even close guesses that could be accurate.