Size
This field has two possible meanings depending on the type of template (comma separated or fixed-formatted).
When working with:
csv formatted output - the output will be truncated to this integer value
fixed formatted output - fields will be expanded or truncated to meet this width, that is, this field in the output will always be X positions in length regardless of the data presented in the field.
Hidden
If you include a value of -1 in the Size field, the term "Hidden" will appear in the saved template definition. Why would you do this? This may be handy if you are using a field as a form of 'working storage' perhaps to perform a calculation where you may want to reference the result in a POST command elsewhere in the template.
Including this value will have the following impact, depending on which type of template you are working with:
csv formatted output - the output will include a comma representing this field but there will be no other value displayed
fixed formatted output - the output will completely hide this field all together (ie it will take up no positions in the output) as if the field were not included in the template. Note that when used with this template type this functions exactly like the HIDE expression.