Invalid Function 'document'

@fredericom, backing up a little bit, you should be aware that you can't use document() to access properties for a just-uploaded file on a SAIL form because the uploaded document lives in a special temporary system folder which is not user-accessible except through a!fileUploadField. The "standard recommendation" from Appian is to verify file properties after submitting a form and seamlessly looping back onto the form again, as Reggie says above, because after form submit you have access to the actual document ID / properties.

Users have discovered a workaround similar to what Venkatesh suggests above - i.e. you pass the local variable containing the newly uploaded file back through a!fileUploadField to get certain Dot Properties that can tell you some information about it, namely filename and size.
However it is important to note that the uploaded document size readout this gives is *rendered as plaintext and abbreviated* - i.e. "5 kb", "2.3 mb", so a simple numeric operator will not work by itself. If one has critical need of doing size comparison on-form, you'd need to write a custom expression rule that takes different size text values and parses them to a standardized value (i.e. multiply out to get bytes for the various different suffixes), and live with dealing in approximate values at the end of the day.

@fredericom, backing up a little bit, you should be aware that you can't use document() to access properties for a just-uploaded file on a SAIL form because the uploaded document lives in a special temporary system folder which is not user-accessible except through a!fileUploadField. The "standard recommendation" from Appian is to verify file properties after submitting a form and seamlessly looping back onto the form again, as Reggie says above, because after form submit you have access to the actual document ID / properties.

Users have discovered a workaround similar to what Venkatesh suggests above - i.e. you pass the local variable containing the newly uploaded file back through a!fileUploadField to get certain Dot Properties that can tell you some information about it, namely filename and size.
However it is important to note that the uploaded document size readout this gives is *rendered as plaintext and abbreviated* - i.e. "5 kb", "2.3 mb", so a simple numeric operator will not work by itself. If one has critical need of doing size comparison on-form, you'd need to write a custom expression rule that takes different size text values and parses them to a standardized value (i.e. multiply out to get bytes for the various different suffixes), and live with dealing in approximate values at the end of the day.