How big would the One Piece planet/universe be if looked from outer space? Since I don't read the manga and I can't remember a lot of it, maybe someone who has more extensive knowledge/understanding of One Piece world can have a rough estimate of how big the universe/planet/ocean is, I'm just curious because all that sea and island being wide apart and we haven't even got to half (or quarter) of it (I guess?).

Would the One Piece planet look similar to earth's size? Or is it more comparable to Jupiter, for example?
The One Piece planet is similar to Earth not only in its Earth-inspired character names. In Chapter 115, Vivi mentions the "Age of Dinosaurs" on Little Garden and Kuzan (Aokiji) has a move called Ice Age, suggesting that there was an Ice Age on this planet. With the exception of the Grand Line, most of the planet experiences four seasons, regular tides, and weather patterns similar to Earth. All of this in addition to regular days and nights suggest that the One Piece galaxy is heliocentric.

However, the One Piece planet has 6 moons, and one of these moons has its own moon (which is large enough itself to be a self-gravitating sphere). Assuming the Ohara citizens were correct about their model of the planet and that the One Piece universe has the same laws of physics as ours, their planet is probably not Earth. Theoretically, it should be much larger but considering imaginary logic, maybe we should just leave the physics up to our imaginations.

I would put the One Piece planet at a similar size to Earth's for several reasons: 


According to the One Piece Timeline (not perfect but probably the best we can get with what we have), it took Luffy and his crew something like 75 days to get through Paradise with excursions (again, I understand that could be spot on or way off, we just can't know for certain). Earth has a circumference of ~40,000 km. The average caravel (Going Merry) speed is let's say 5 knots with a Nami navigator and Grand Line conditions. A Franky-built brig (Thousand Sunny) averages let's say 10 knots because Franky is awesome and Nami is navigating. So that means the Straw Hats traveled Paradise at an overall average of let's say 7 knots. 20,000 km / 7 knots is ~65 days without excursions and the Straw Hats knock out most of these excursions in a day or two.
Apart from the Grand Line, the One Piece planet has Earth-like weather patterns and four seasons per year. Considering Luffy supposedly has a Gregorian calendar birthday (Cinco de Mayo), there are 365 days in said years. Oh, also, he weighs 64 kg and that's a reasonable Earth weight. He is really strong though so maybe it's a bit smaller and that's why everyone can jump really high and Luffy only weighs 64kg, apart from "because reasons," which is probably how we should be approaching this question. Now, correct me if I'm wrong but I do not believe either of these measurements are technically canon, so take all that with the same grain of salt with which you are probably taking this question and answer.
The deepest part of the ocean we know about in One Piece is just below Fishman Island, which lies 10km below sea level. Not-so-coincidentally, the deepest depth of Earth's ocean is just over 10km. Since the One Piece planet supports land and water on its crust similarly to Earth, it probably has a similar geological structure to Earth. With identical water depths, I think I can rest my case here.