A field trial (also field test or field experiment) means a test or experiment performed under conditions which are more natural and less controlled than in a laboratory experiment.
It does not need to involve a literal field (paddock). For example, this quote is from an example of a field trial in telecommunications which involves providing equipment to customers:
100 mobile and 100 fixed line customers in the St Newlyn East area of
South Newquay, Cornwall will be included in the trial.
Apparently a test at BT's labs in Adastal Park in Surrey is already
underway, but the field trial will give a much better measure of the
The definition of a small-scale trial is not very clear in many cases. Here, it probably means "one to a few fields" - not necessarily that the fields themselves were small. A trial on one large field might still be considered a small-scale trial.