It can be calculated from the difficulty for the most recent block. To get the difficulty, first get the "baseTarget", which you can see in the output of the mining window. Or, you can go to http://burst.cryptoport.io/, and get the details on the most recent block. The equation for difficulty is:
(This is different than the Difficulty reported in the BURST web client, which is actually reporting the Cumulative Difficulty of all blocks)
Then, to calculate an estimated total size of miner's plots. The equation is:
Based on the current baseTarget of 12795815, the estimated total plot size is 1432 TB.
When calculating the total plot size, it might be better to take the average baseTarget over the past 10 or so blocks in order to get less variance in your calculations.
Thanks to uray, in this post for clarifying these equations (you can also see where these equations come from in that post).