Which is the smallest or quietest services?
Old Inns is tiny - but does it count?
This question is more complicated than it ought to be.
If we take "motorway services" to mean any service station that is signposted from a motorway (as we do), then Appleby Magna (on the M42) and Old Inns (on the M80) both have little more than a petrol station. This means they are likely to be the smallest and the quietest; Old Inns is marginally smaller.
The problem is neither of them were intended to be a motorway service area, they just became one by a recent fluke of history. As a result, people tend to treat them more like A-road sites (which is what they were designed to be) - and A-roads have hundreds of tiny petrol stations!
Of the motorway services that were always intended to be one, Severn View on the M48 has a tiny building and low trade, but is built on huge grounds. The winner is likely to be in Scotland: Kinross on the M90 is very quiet, and the building at Bothwell on the M74 is smaller but built on larger grounds.
See also: Biggest