Hi!
Be more specific. This type of connection for roofs is normally used for compensation of temperature induced deflection. If this is your case, you should compute the maximum of such a deflection and take it as a limit. If you can't give the structure as much sliding freedom, you must perform an analysis in which extra stresses due to tempreture is taken in to account.
For bridges, however, this type of connection is used to prevent fatigue due to the vibration. In this case you should have no restraint against sliding.
Theoretically, amount of longitudinal deflection would be a fraction of vertical deflection. In trusses, however, the vertical deflection is limited, so you shouldn't be normally worried about horizontal one, except you are dealing with a special problem.
If none of these problems you are facing, simply change the sliding type to a hinge one. It will reduce the size of the elements, but you should be aware of lateral reaction on the supports that could be as high as 1/10 of vertical reactions, or even greater.
Good Luck