The sizing question is different on a small plot. On a large garden the question is how much space to build. On a small garden the question is how much of the garden to give up for the build, and this is a harder number to land on than many buying guides seem to suggest.
A garden room that takes up a third of a small garden alters how the entire space works, not just the area that it occupies. The remaining space is often too narrow to be of any use, and especially not a proper sitting area.
Positioning is important here too. Building as close to a fence line as possible in order to maximise open garden space often means checking permitted development rules on distance from a boundary. Many people are surprised to find out how much those limits are actually restrictive.
Glazing on smaller structures pays off more than it does on larger structures. Floor to ceiling glass on a smaller garden room makes the space feel like a garden extension, especially if the existing garden space is already small.
Expect a small well-insulated garden room to cost around £15,000. Cost increases dramatically with size. Think about pricing a smaller structure that has better glazing and insulation against daily use, rather than defaulting to the largest one that a plot can technically accommodate.
See also: Conservatories — Insulating & Using a Small Conservatory Year-Round