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The security margin model has a few methods that can help determine the margin requirements.  The method GetInitialMarginRequiredForOrder(Security security, Order order) gets the total margin required to execute the specified order in units of the account currency including fees. If there is no order, a more general method GetInitialMarginRequirement(Security security) gets the percentage of an order's absolute cost that must be held in free cash in order to place the order.

Gurumeher Sawhney  thank you! 

But as I understand GetInitialMarginRequiredForOrder is a protected method and can only be called within a class itself or its ancestors. 

At the same time, Securities[symbol].MarginModel property is of IBuyingPowerModel type. 

So to use any method of SecurityMarginModel interface implementation I need to cast that to a proper type then call a public method? 

In live trading mode - paper account through IB - how do I need to cast the MarginModel property to call the public methods, I wonder?

You're right, since Python is not strongly-typed it was easier to access those methods. When trading equities the initial margin requirement is simply 1/leverage. The IBuyingPowerModel type has similar methods such as HasSufficientBuyingPowerForOrder(SecurityPortfolioManager portfolio, Security security, Order order) and GetBuyingPower(SecurityPortfolioManager portfolio, Security security, OrderDirection direction) which will return the values that are needed.

Hmmm, for position sizing I'm trying to get the maximum qty of a security I can trade at a specific (limit) price - which I would assume to be: 

security.MarginModel.GetBuyingPower(...) / securityMarginCost

where 

securityMarginCost = security.Price * security.SymbolProperties.ContractMultiplier * initialMarginRequirement

The problem, as mentioned above, is both GetInitialMarginRequirement and GetInitialMarginRequiredForOrder were made  (why??!) protected [in .NET]

 the GetLeverage method suggestion above is incorrect  as GetLeverage returns 1/MaintenanceMargin, not InitialMargin. GetMaximumOrderQuantityForTargetValueResult targets an account %ge and depends on existing holdings, market price, etc which doesn't really help in this situation.

Is there a canonical way to determine how much of a security I can buy at a given price, taking into account buying power and security leverage?

search for CalculateOrderQuantity maybe that helps

Thanks Michael!
For posterity, OpeningBreakoutAlgorithm.cs provides an example of risk based position sizing with CalculateOrderQuantity, which indirectly uses GetMaximumOrderQuantityForTargetValueResult with a provided max leverage.
The current market price is used determine the allowed size, but that could be adjusted for a limit price if significantly different, I guess.