Is there a way to find out what the original size of the Sun was?

Here you go, it is 20% larger. >>>>>>>>>>>>> https://www.spacecentre.nz/resources/faq/solar-system/sun/getting-bigger.html

@Tropical_Willie Ah, so that’s a lot more evidence that Venus was more Earthlike billions of years ago. I’m really not sure if Mars ever had life on it, then. Do you think Mercury could have been more Earthlike in the beginning, before whatever struck it caused it to lose a lot of its diameter?

Mars had liquid water on it in the past. Losing its atmosphere was a result of several factors. Venus was probably earth like once.

During the Cryogenian Period (between 750 and 600 million years ago) the Earth was completely frozen over. It seems logical that during that period Venus and Mercury would have been cooler as well and more hospitable for life to exist.

First you must define what time point is “original”.