SSRS UIBuilder class - dialogField loses its displayLenght property after initial running, help!

Hi, I;m creating a SSRS report in DAX 2012 using DataProvider, Contract and UIBuilder class, in the build method of the UIBuilder class I have defined the dialogField that is "linked" to the parameter in the Contract class:

public void build()
{
contract = this.dataContractObject();

commodityField = this.addDialogField(methodStr(myContractMethod, parmCommodityId), contract);

commodityField.displayLength(25);

}

It works, since the initial time I run the report the field is the proper size, when I generate the report, the field is the default length, I don't know why, I already setup the field in the postRun and postBuild methodsbut it does not work, everytime that I generate the report, the field goes back to its original length, any help? thanks!