This is a fantastic question, and living, walking proof that some of the simplest questions to ask are the most complicated ones to answer. I'll put the short answer up now, but I've been writing up a more detailed one which is available on request. The short answer is, size isn't really the trigger. The control is all about DNA. Size certainly has limits. A cell needs to have a minimum volume of ~0.03 micrometers cubed, and it can go to a maximum volume of ~50,000 micrometers cubed -- and this is based around the simple need for a minimum number of hydrogen ions within the cell (on the small end) and the maximum area for diffusion of gases in order to power chemical reactions on a practical timescale (on the large end). However, there is a great deal of difference between 0.03 and 50,000 micrometers^3, and within those wide limits, each type of cell has a "typical size" which it tends towards, and rarely exceeds. So it is immediately evident that size itself doesn't impose the limit; otherwise *everything* would just grow to the maximum possible before dividing, and they don't. They hit the typical size, and then, if they divide at all (many cell types have long periods of time in which they do not divide), they divide there. The actual trigger for cell division is linked to the replication of DNA. The first step for successful cell replication is duplication of the cell's full complement of genetic material; and such duplication is very costly to the cell in terms of material and energy required, so the cell does not commit to doing so until it has received signals that there is plentiful nutrition available in the area (in the simplest and most straightforward case). Prokaryotes and single-celled eukaryotes rely on such signals from their environment to control replication, and there are generally both receptors on the cell membrane and enzymes within the cell itself which respond to temperature, pH, and levels of nutrients to tell the cell whether it is in a good environment to replicate, and in a good environment replication is promoted and in a bad environment replication is repressed. In these simple systems, the cell is committed to dividing shortly after its DNA is replicated, as the replicated DNA forces the cell to grow to accomodate it. In multicellular, eukaryotic organisms, there are two sets of signals: one to tell the DNA to duplicate, and one to tell the cell itself to grow and divide. There is a gap of time before each event, which can be considerable (sometimes the gap is so small as to be nonexistant, as when an embyro is developing; sometimes the gap is not just years, but decades, as in the replication of certain types of nerve cells). The cell can replicate its DNA without *having* to grow the way prokaryotes do, because eukaryotic cells are generally larger to begin with and there is generally plenty of room for the genetic material. So there are two separate stages to the commitment (although once the commitment for cell division, rather than just DNA duplication, is made, the cell can't stop halfway). The signals that each type of cell responds to are often highly individual, taylored to what the cell is and does, and the signals themselves may be the simple environmental presence or absence of something (as above), or they may be very specific chemical messages "sent" from neighboring cells. But there are two specific classes of chemicals which respond to whatever the signal is and work to regulate cell replication -- these are called "cyclins" and "cyclin-dependent kinases", and the scientists Hartwell, Hunt and Nurse shared the Nobel Prize in 2001 for isolating these molecules and describing how they work. The actions of these molecules are themselves often controlled by the basic mechanism of adding or removing phosphate groups from specific locations, something that is done by other enzymes and enzyme complexes in the cell reacting to the signals mentioned above. The study of eukaryotic cell replication and division is a large and fascinating one. It's a bit difficult to summarize coherently, but I hope I've given you a better idea of it.