It can be difficult to decide on an excavator size, considering the drastic range in size among these machines. There are a few key factors to discuss before moving forward with your decision:

Budget - how big of an excavator are you willing to pay for? Larger models with more power are going to cost you significantly more than a smaller model
Worksite - what kind of terrain are you working on? Are you indoors or outdoors? Do you need to worry about inclement weather or tight corners? Pick an excavator size that plays to your jobsite’s advantage.
Needs - what kind of work needs to be done? Even outside of the attachments available to you, the size of the excavator limits the kind of work it can do. If you need to haul heavy materials or move large objects, a smaller excavator might not be able to do the job.