Hello,
The 
BizForm control is based on the 
BasicForm control that provides a property called 
FieldControls. It is a standard 
HashTable that contains all controls as 
FormEngineUserControl. 
However, if you need only the entered data, it should be possible to use the 
BasiForm.DataRow property instead, for example:
DataRow row = BizFormControl.BasicForm.DataRow;
string email = ValidationHelper.GetString( row["EmailField"], String.Empty );Moreover, the BizForm control provides a few handlers you can use to access the entered data, such as OnAfterSave, OnBeforeValidate, OnUploadFile, etc. 
I would like to point you to the following knowledge base article that demonstrates how you can use the OnUploadFile handler to validate the size of uploaded file: 
How to validate the size of uploaded file in BizFormFYI: BasicForm.DataRow has been replaced with BasicForm.Data in Kentico CMS 6.0
Best regards,
Michal Legen