Occasionally people find that they cannot save form submissions with upload files greater than a certain size, often 1MB.
CFDB does not limit the size of file uploads, but other things do.
Check The Form Definition
First, check your form definition in whatever front-end form plugin you are using. There may be a setting to set the file size limit (such as with Contact Form 7).
Check The WordPress Memory Setting
Your server should much more memory than the max size of your upload. Check your WordPress memory settings.
Check MySQL max_allowed_packet
The file binary gets stored in the database. Often it is the MySQL DB that is rejecting the insert of a large Binary Large Object (BLOB). This is fixed by changing the max_allowed_packet setting on the MySQL server. You may need to contact your hosting provider to do this for you.