Which is the smallest or quietest services?
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.
A special mention should be given to Cullompton, which is the smallest service area to be on a very busy motorway - the M5, and also to Pease Pottage which started out tiny but has become huge.
See also: Biggest