Is there any way to know the amount of bitcoin wagered on PrimeDice in last X hour (e.g. 24 hour), where the minimum size of bet was Y BTC (e.g. 0.0005 BTC)?
No, i don't think that kinda of data being available publicly. you can get someone to code a bot for you to monitor the bets and log it for you or ask stunna.

may i ask you why do you want the wagered amounts by bet size?