There are some codes to calculate the size of tiddlers like the plugin size from @telmiger or the solution by @EricShulman or the code is displayed in edit mode from @linonetwo. I am sure there are other solutions out there I have not seen.
One simple and close to real size is the below WIkitext
Create a tiddler, and tag it with $:/tags/ViewTemplate/Subtitle
Put the below WikText inside body (text field and save)
Open any tiddler and see the size in kB at the bottom of tiddler.
The tiddler is tagged with $:/tags/EditTemplate and has a list-after field set to $:/core/ui/EditTemplate/controls, so that the output appears below the "Draft of ..." title while editing a tiddler. The output shows a summary with the total lines/words/bytes for the current tiddler, followed by a down-arrow. Clicking the summary opens a popup containing a table that shows the lines/words/bytes for each field.
As a result of @Mohammad’s OP, I realized that I wasn’t including the size of the field NAMES in my size calculations! In response, I’ve updated several places where this type of calculation was being done. The following tiddlers have changed to more accurately reflect total tiddler sizes: