You'll want to create the following new column:
cnt_p_rfi_bb_total: sum(if[tourney_holdem_hand_player_statistics.flg_p_first_raise AND tourney_holdem_hand_player_statistics.flg_p_open_opp, tourney_holdem_hand_player_detail.amt_p_raise_made / tourney_holdem_blinds.amt_bb, 0])
Divide this column by cnt_p_raise_first_in in your stat's value expression and you'll see the average bet size in big blinds that the player raised first in.
For more on creating custom statistics and reports please see the
Tutorial: Using Custom Statistics and Reports.