Well, you can change the shape & size of an iFeature both at the time you first place it, and afterwards, as long as you have that flexibility designed into it. iFeatures have their own set of parameters that they use, which can be named appropriately within the definition, and those parameters will be created within the 'ModelParameters' of your current model file when you insert the iFeature. The names of those new model parameters will start with the name of the iFeature (as it appears in your model browser, with : and digit(s) included), followed by its original name. I do believe it is possible to accommodate changing those parameter's values/expressions/equations by code. I don't have a current working example of driving placed iFeature's parameters from Excel, but I don't see why it couldn't be achieved by code if set-up properly. Of course iFeatures are similar in some ways to iParts & iAssemblies too, in that they may have a table which can hold a great many variations, and that table can be edited with Excel if needed, but they do not carry this table with them into the model browser where you place them, like iParts/iAssemblies do. To change the iFeatrue's changeable aspects manually, you simply right click on it and choose Edit Feature, then it will show an appropriate iFeature dialog for you to change those things within.
Wesley Crihfield

(Not an Autodesk Employee)