To limit the size of the uploaded file, use Server Validation included with CSS Form Builder.
Use Number validation, Select the File Field File Size from the bindings panel, and set the maximum number in the number validation settings to the maximum size in bytes you wish to allow. For 4MB use 4,194,304.
the problem you are having with uploading larger files is due to php server settings. See the following page for details on php.ini settings that control the allowed size of uploaded files: