Happymidgetpaul,
I saw a pic of your mangrove, Great looking lil guy
I was looking through my personal photo collection to see if I could get an idea of average size. 4 feet is the “average” adult but I have personally seen 2 that were larger than my own who is 5 ˝. Ft. Remember that adult is kind of a loose term because monitors continue to grow as adults. My monitor was an adult at 3 ˝ feet, yet he continued to grow slowly but noticeably until the age of 9. The last 2 years he looks pretty much the same. Your little guy should grow around an inch a month if he has proper food, humidity, heat, comfort, exercise, etc. He could grow as much as 2 inches a month. Mangroves are very active and like to climb and swim. When you do your first cage upgrade I would plan for 2x what you think he will need. Mangroves are very smart and WILL ESCAPE if there is any possible way. They can fit into some of the most unimaginable places so be aware. Make very very sure your enclosure is secure. Bugs are a great staple diet for your little mangrove. I don’t know where you are from but if you can get little crayfish that will be a favorite! Also, little snails are a good meal if you are sure they have not been poisoned. Fertilized quail eggs can be purchased on ebay, he would be happy to eat those too.
Anyway, good luck with your little guy!
-H-
