If the signature I have forwarded in the form seems to be smaller than that in the sample form. Is there any chance of getting rejected. Will there be any more date to check the details well before the final editting.

there is a very small chance of rejection in this case... as long as the signatures are properly done irrespective of the size.

if there will be any editing needed in your form, you will most likely receive a email from IIT for that purpose..