Heill Dan!

> I had a bit of trouble understanding the usage of "sem mest-",
> could you elaborate on this idiom?

Normally it means 'as much as possible' but it seems to be
rather overused in Færeyinga saga and is perhaps best translated
with 'large' some of the time.

I do not know whence that idiomatic meaning arose. Perhaps from
(still used) constructions as 'sem mest hann má' = 'as much as
he can' where the latter part comes to be dropped.

Kveðja,
Haukur