I’m sure there is an equation to determine the mass based on it’s speed and distance from the host star...............
The plan is to land on the nearby sun to study the planet more closely, but do it at night.
Using spectroscopy you can measure the velocity oscillations of the host star and planet in the direction of the earth very very precisely. Based upon this you can determined the planets effect on the motion of the star and that gives you a measure of the relative mass. The masses of stars are generally determined from their luminosity since there is a very strong correlation between mass and luninosity for “main sequence” stars.
You can determine mass from velocity and distance but size would be difficult without knowing the density and elemental composition. Most matter in the universe is hydrogen and helium and that is most of what is out there. Something like 1 or 2 percent of the universe is anything other than hydrogen or helium and the majority of that is oxygen carbon and nitrogen. All the other elements are less abundant according to their nuclear composition with heavier elements being more and more scarce as they increase in mass. There is also a tendency towards elements with an even number of protons.