Searches are generally driven by the amount of people, rather than a specific amount of space. Office providers offer different amounts of sq ft per desk/person, and offer various amenities outside of the office that you wont need to have within your demise.
A private office in a serviced office could take up the same amount of space as 3-4 people, so a few of these can dramatically increase your costs
Does each individual require their own dedicated desk, or can some share a workbench, hot desk style. This is a style of working growing in popularity as companies embrace flexible working arrangements
Depending on the amount of employees you have, you may wish to factor this into your search, or simply use the communal breakout facilities provided and save on your costs.
If your employees need a private area to speak on the telephone away from others then this may be important.
Haven’t gone paperless yet? A filing cabinet(or two) can take up the same space as people so will need to be factored into your search.
Paying for your own dedicated meeting room will increase your monthly costs, so unless you use a room multiple times a day then providers can generally offer easy regular access to communal meeting rooms at a reasonable cost.
Not all providers can offer this, so make it clear from the beginning what the specific needs of your business are.