Question
question .. what size arrangment should  be used on a wedding reception table  measuring 38 inch round for guest sitting

Answer
Dear Kathryn,

As 38" isn't very large, I usually go with something tall rather than something wide.  A small 6" arrangement when set atop a base (I often use candlesticks) is something that would work very well.

Another way to go is to use a cluster of small clear bud vases, some tall, some squat, with a single bloom in each.