Decrease. The support pulls the wrist away from the ball, decreasing span length.

Wrist supports are like cars -- they come in many shapes and sizes. You can't talk about wrist supports in a generic fashion and come up with a single magic answer related to span.
 
Some wrist braces result in almost no change to span, while others affect span noticeably. Work with your pro shop guy to determine the effect of any support you're considering. 

Steven is correct it all depends on the type of brace it is.

They can either Increase or Decrease span or have essentially no affect. Beside the wrist, it's also how they hold the fingers. (usually the extended types)

When they have straps that hold the fingers back and restricts their movement, it opens the hand more and lengthens the span.

One like the old Robbys Gladiator typically stretched the span, because it held the hand more open.

Ones like the small Robby's “Leather Original” wrist support, have virtually no affect.

The main thing is to have your span remeasured if your going to use one for a prolonged period.

Please excuse the Hi-jack of the post-  Will a Wristband assist in keeping your hand under the ball - making it easier for you thumb to come out first prior to roating you fingers.

i noticed the better bowlers cupping thier wrist-- i am unable to do that -- is the wristband the answer or helpful

Thank you all for your advice and information