Yup, I can say for sure that they get to be bigger than your thumb as adults. They can be kind of pretty, with red/orange/ yellow and black markings. But they'll eat your garden.
I think probably if you don't want to squish them, putting them in a jar with soapy water and/or bleach is less messy.
Also, a birdfeeder in your yard will attract more birds that will also eat the lubbers. At least when they're small. When they get big, they're slow enough that your dog or cat can catch them.
