Any worthy E.T. must be able to hold up his side of the conversation. So we can assume a high degree of complexity. That rules out the possibility that our alien pals will assume the shape of a malevolent mound of Jell-O, such as confronted by Steve McQueen in The Blob. But of greater concern, the bulk of its protoplasmic innards would be situated far from its outer membrane. This would fatally hinder the exchange of gases and nutrients needed for survival.
Humans are relatively new to the universe.
Other races, as we imagine them, would be much more advanced than us. Click to see some Hollywood aliens and why they're not too realistic. There's also a limit on how small a cell can be. Mycoplasma bacteria, microscopic organisms that SETI astronomer Seth Shostak muses about what E.T. might look like. A substantially smaller cell would have a volume too puny to support the necessary chemistry of existence, a fact that also applies to aliens.