A Ceiling, Not a Size
No more than 5cm by 5cm.
Most authorities publish an exact size, a few publish a range, and Barbados publishes only an upper bound: the photographs should not be more than five centimetres by five centimetres.
That is a genuinely different instruction. Anything square and smaller is inside the rule, which is unusually forgiving — but it also means the department is not going to tell you what it actually wants, and a photographer used to a national standard elsewhere will simply give you theirs.
The department does not publish a head height either. This page therefore shows the 5x5cm square as a working frame and does not present a face measurement as a Barbadian figure, because there isn't one.