There is no simple answer to this as it will vary wildly, but to give a ballpark figure, currently transactions average around 5.6kb (I got this by averaging the last 50 blocks from [monerblocks.info](http://moneroblocks.info/stats)), and with the current rate of transactions taken from moneroblocks.info we can see it's ~2 transactions per block. Given that the block transmission is every 2 mins, that means it grows ~4.03 megabytes per day simply from transactions based on current numbers. The current blockchain size is around 7.5Gb.

Papa Lazarou made a more accurate representation of monthly block growth in megabytes.

         Jan      Feb     Mar    Apr      May     Jun     Jul     Aug     Sep     Oct     Nov     Dec        
    2014                          15.25   159.6   424.98  202.63  227.67  155.52  102.11  83.77    103.33   
    2015  74.76   71.97   100.49  69.54   65.58   65.03   65.96   67.09   61.86   67.82   55.47      65     
    2016  77.37   69.88   131.46  107.59  78.63   87.72   84.67   136.9   170.21  143.24