That depends on the types in your protocol, but I’d usually go with either directly matching types of equal size or the next-largest type that can fit your elements.
70/80 bits is a very awkward format to receive and sounds like that’s a composite type (i.e. a type made up of multiple smaller, more primitive types). I’d unpack that from its compressed form to individual fields (or even create its own type for unpacking that into), as unaligned accesses to memory are extremely slow and constantly shifting/masking operations are not really conductive to high performance (this does depend on how much work you do on your received data, but it’s fairly straight forward to implement and easy to use).