Geometry time - since:

area = pi * radius^2

diameter = 2 * radius

A bit of calculator work indicates:

AREA Diameter AWG wire size

.75 mm^2 .977 mm between 18 and 19

1.5 mm^2 1.38 mm between 15 and 16

2.5 mm^2 1.78 mm between 13 and 14

Common AWG size to diameter in mm and cross-section area in square millimeters (mm^2):

18 ga = 1.024 mm dia = 0.82 mm^2

16 ga = 1.291 mm dia = 1.31 mm^2

14 ga = 1.628 mm dia = 2.08 mm^2

12 ga = 2.053 mm dia = 3.31 mm^2

10 ga = 2.588 mm dia = 5.26 mm^2

Derived from a table giving wire size in mm diameter from _Pocket_Ref_ , Sequoia Publishing (a handy book to have...)

As for strippers, try to find one of the "universal" types with jaws that grip the insulation and pull it apart...