Is there a way to increase the default size of the Field Help window?

No it is not possible, system default no way to change. Can try with DOM or something like that but will not recommend it.

It won’t produce immediate results, if ever, but you can search for an existing Enhancement and upvote it. Or create one. I’ve often thought it was annoyingly small.

It wouldn’t be too difficult to hack this using a userscript with a browser extension such as Tampermonkey.

All you would need to do is overwrite the global function `nlFieldHelp`, and change the numbers 400 and 150 in the third from last line to your desired width and height.

It’s not recommended, but probably worst case is it could break the field help in a future release and you would need to deactivate the user script.

function nlFieldHelp(windowTitle, fieldId, flhId, fld, helpTopic, company, formType, formTemplateId, record, isTransactionForm, formTitleTranKey, formTitle, fieldLabelTranKey, fieldLabel) {
  //See FlhUrlParameters.java for param definitions
  var url = NS.Help.getFieldHelpUrl({
    id: fieldId,
    flhid: flhId,
    parameters: {
      "topic" : helpTopic,
      "c" : company,
      "ft" : formType,
      "ftmp" : formTemplateId,
      "r" : record,
      "tr" : isTransactionForm,
      "fttk" : formTitleTranKey,
      "ftt" : formTitle,
      "fltk" : fieldLabelTranKey,
      "fl" : fieldLabel
    }
  });
  nlExtOpenWindow(url, 'fieldhelp', 400, 150, fld, "no", windowTitle, {}, fld);
  return false;
}