Lots of different species are called Mud crab, including a fictional species called mudcrab in the Elder Scrolls game.
Scylla serrata can have a shell width of up to 24cm and can weigh up to 3.5kg. This mud crab is also called a mangrove crab.
Austrohelice crassa gets to just 4cm. This mud crab is also called a tunnelling mud crab.