Since a Kudu table is distributed across tablet servers, the total size of the table is the sum of the sizes of its tablets. /metrics has an entry for each tablet, which will list which table it came from and the on-disk size of the tablet, so you can roll up all these numbers to compute the total size of the table. Kudu does not do this for you, but metrics software should be able to.
Also, keep in mind that the on-disk size metric for tablets doesn't capture the whole size of the tablet. See KUDU-1755