So, you have said you are using Crystal for your template but you have not indicated which form you are printing, so let's assume you are printing an 8.5x7 cheque for Accounts Payable.
You have setup the Custom Form in the printer drivers.
Then you created a new printer driver and in the properties of that new driver you set the Advanced Options Paper Size to the new form.
Then in Simply you went to Setup, Reports & Forms, Cheques, selected the bank account and at the top under Payment Cheque Settings, you set the Printer to the new driver.
The form type is set to Custom Crystal Reports form (change the radio button to Custom Forms if it is not already set.
Change the Description to User-defined form and Browse for the form you created in Crystal.
If the form is an invoice, then change any word that refers to Cheque above, to Invoice and follow the same instructions. Is this what you did and it still doesn't work?