I once encountered one around 10 inches.
The worlds biggest praying mantis is recorded at 18 inches
The biggest pray mantis iv ever seen was about 20cm
About 4 in.-5in. If you find a foamy liquid on a plant or twig, they are praying mantis eggs, you can raise them like that.
Streach your pinky and your thumb apart this is a little over as how big they will get
Mine grew to be about three and a half inches before it died.
I saw one in Panama going through jungle warfare that had to be at least 9" long.
The biggest Praying Mantis which I come across is about 10 cm to 15 cm in length. As it is very difficult to find information about this on internet; therefore, I am sure that they are much bigger than this.