It is interesting that amphipods are found in Mexican amber because they are generally associated with an aquatic or semi aquatic habitat. They do occur today in warm, moist terrestrial habitats and that is probably the type of environment utilized by the genus found in Mexican amber, Palcogammarus.
The piece of amber is not real large, about .75 x .3 x .3 inches. The rolly-poly is perfect. Probably the best I have ever seen in any amber.
If Isopoda is your thing, you will love this piece, it is great.