The only way to get this information is to research the ID of potential companies providing such a service (using Commercial Directories), then contact the companies direct and ask the supplementary questions. They would be under no obligation to tell you. That is the way I would do it for the UK - I doubt that it is even possible for the USA - it's an order of magnitude in scale difference in overall size compared to the UK.
An observation that I afraid means I think your approach to this is somewhat misguided:
whether or not they supply to SME or larger businesses doesn't seem relevant to your business plan and it certainly isn't going to be sourceable from public information. If I ran such a business and someone phoned me out of the blue noseying around for such info I wouldn't tell them. And anyway, I doubt that this segmentation constrains such a business because quite small businesses are able to sell their services to other businesses of ALL sizes - and especially quite large ones.
Time to go back to Business Link or whoever elese is advising you about how to do a Business Plan - you are too far into the weeds.