Fortic (think thats how it is spelt) is IMI Range Boilers combination
This consists of a hot water tank with a combined header tank fixed on top.
There are various capacities and sizes plus some have immersion heater
holes.(either top or side)
They used to make Direct , Indirect and Primatic ,some were foam covered
.(the Herculag range).
Today many different manufacturers produce combination cylinders but the
name Fortic has stuck.
As for size ,when IMI were in full swing they would make you any size you
liked (the largest I saw was 48" dia by 76" high.)