There is no requirement that the overseas business transferring the employee over to the US has to be a particular size.  However, very small businesses with say one or two staff may have difficulty coming under the L1 intra-company transfer visa.  For a very small business you may have difficulty showing that the business will remain trading outside the US while the L1 visa applicant is in the US.  For very small businesses it may also be difficult to show that you meet the requirements as an L1A visa Executive or Manager outside the US for one year in the last three years.