Standard florist wire is usually enameled green and comes in 18 inch lengths. The painted wire is resistant to rusting (a common hazard when using on flower stems that come in contact with water).
The gauge of a wire determines the diameter of the wire. The higher the number - the thinner the wire. Thus, 18 gauge is a heavy wire, intended for support of thicker flower stems or very tall stems that need additional support (as in an altar bouquet). This wire can bend or crush delicate stems if you are not careful.
It comes packaged 25 pieces to a sleeve.
This 21 Gauge wire is my favorite personal choice. It's strong enough to support most flower stems, yet flexible enough to use on pew bows with lots of loops. This package has 40 pieces of wire.
26 Gauge wire is easy to bend and is a little thinner than the wire pictured above. It, too, is all purpose wire - but may not support the heavier flower heads. This wire comes in 60 piece packs.
A 30 gauge wire is very thin and reminds you of the wire used for beading crafts. You can use it for supporting tiny stems such as lily of the valley or stephanotis. This is often a favorite of florists for using on corsage or pew bows. You'll find 70 pieces of wire per sleeve for this gauge.
All gauges are packaged in assorted pieces, generally determined by the weight rather than by the piece. Florists tend to buy wire in bulk, usually 1000 pieces or more. These boxes are very heavy to ship. I sell the packaged wire, making it more cost effective for a bride with a smaller project in mind.