happy tea,
you can check with your coordinator on the size of the table. if your venue organiser does ROM often, they should know and be able to quote you the dimensions.... you should find out early so that if you think it's too crowded, can ask for a spacious one...
for decorations, you can consider getting ivy leaves and arrange it on the border of the table, together with the rose petals on the table... that should be simple and nice... good also if you can get a small arrangement on one corner and your ring pillow in the middle....
can also consider tie-back for the chairs with floral decorations.