There is always a limit to how large an extension on a house can be. If you extend under a householder planning application there is no set size an extension must be, but there is more emphasis on making sure it is appropriate to the size of the plot, ensuring the extension is subservient to the original house. You will also have to consider neighboring properties and ensure you do not inappropriately block their light or obstruct their views and outlook.
Under permitted development the rules are much stricter. There are set depths that extensions can be, for example, a single storey extension on a terraced or semi-detached house must not extend beyond the rear wall of the original house by more than 3 metres; on a detached house it must not extend more than 4 metres. When a house is not on designated land, these distances are increased to 6 and 8 metres respectively. The location of extensions under this type of application and where you are building them is also important. No extension can be built forward of the principal elevation. If you have an extension already in place, then speak to an expert to assess whether you can apply for additional extensions under permitted development rules.