I've had many questions regarding the wire & bead sizes.
Since I live in Holland, we use a different metrics system than the US.
Dutch shops sell wires in millimeters:

Brass wire: 0,30 mm
Green/brown : 0,31 mm

It seems that the wires go somewhere between a 28 and 29 gauge.
You could use the following sizes:

Brass wire: 29 gauge
Green/brown : 28 gauge

Here's a little conversion table with a range from 25 - 35 gauge so you can decide for yourself what's the best size to use.

American Wire Gauge

              = best size used for creating beaded flowers
 

 
Beads have their own sizes.

Here's a picture with the beads sizes and the sizes in millimeters.

 

 
I mostly use beads size 11/0.

You can best combine brass wire (0,30 mm / 29 gauge) with beads size 6/0 - 15/0.
Colored wires (0,31 mm / 28 gauge) go best with beads size 6/0 - 11/0.

Size 15/0 is fairly small and the brass (or silver) wire fits through it twice (although it gets narrow).
With the green (brown or white) wire it gets difficult. 
These wires are painted, that's the reason why they're somewhat thicker.
It's better to use beads size 15/0 on a single wire like basic techniques like #2, #4 or #5.

Just use your creativity and try different sizes beads and wire.
In time you'll get the hang of it and you'll know exactly what size beads go with what size wire.

13.779528

15.748031

17.716535

19.685039

23.622047

27.559055

31.496063

35.433071

39.370079