Assign the area to category value

Hello,

After clumpping (using r.clump) the map, I got many continuous polygons. Now I
want the area of the polygons to be the category value of the corresponding
polygons so that I can choose polygons according to their area. I forgot the
appropriate program to fulfil the task and couldn't find it. Would someone
please give me a hint about this?

Many thanks in advance,

Houxiang