Tag Type C++ code (built in to libHForm/scan_body.cc)
Attributes var=varName Required. Sets the variable name whose value we want to set with this text entry field.
size=integer Optional. Sets the number of visible spaces in the box (but not the maximum length of the variable.
Contained Tags None.
Related Tags This tag must appear in the Body section.

Here an entry blank for a 5-digit ZIP-Code is given, with a blank 6 spaces wide (so the text in the blank doesn't scroll horizontally when the cursor moves to the last position.)
  {Entry var=zipCode size=6}