Arras wrote:
I want to get some Phillips screwdrivers for very small screws.

Most suppliers quote something like this:
"#0 and #00".

However others quote things like this:
"7 Heads Phillips 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0".

How can I work out which is equivalent to which?

Are you sure you want Phillips? They are not the same as pozi-drive
which is possibly more common in UK use.

The head of a pozi "normally" has an x on the head rotated 45 degrees to
the main driving faces.

The tip angle of the two standard are different and it becomes quite
critical on the small sizes where the wrong screwdriver can easily wreck
the head of the screw.