Assuming that the balance of the 14k in your account after your purchase will not be used for other trading (that is, it's dedicated to maintaining margin levels against your silver futures contracts) then a very useful approach is to calculate what your margin liquidation figure would be for various sizes and then make your determination based on that. For instance, if you are trading thru IB they require $1,500 per YI on the eCBOT (other brokerages may have different margin figures). Using that margin number along with an initial 14k account and the closing price on Friday, if you bought:
1000 oz (1 YI) your margin liquidation would be about 4.37
2000 oz (2 YI) your margin liquidation would be about 11.37
3000 oz (3 YI) your margin liquidation would be about 13.70
4000 oz (4 YI) your margin liquidation would be about 14.87
Obviously 4 YI would be too risky as it's very possible the price could go down to that figure wiping out your account. 3 YI is a little safer but still very risky as that price could also be hit on a whoosh down (anyone who trades silver knows those yee-haws do happen, even moreso in the YI). 2 YI provides a few dollars more comfort zone, and if you don't think price has a realistic shot of getting to 11.37 that might be the size to consider to get the most bang for your buck. 1 YI for practical terms should be about as safe as safe could be (outside of a hedge as SethArb described) since that would require a 74% drop from current depressed levels which is beyond unlikely as we are still in a silver bull market. Hope that helps.