Hello,

From what this looks like, you're gonna gave a fixed zoom later on in the game. If you don't, the following might not apply as closely, but I'll try to make it worth your time.
Sprites do always look the best unscaled, so I'll talk about how to get the size for an sprite in original resolution 1:1.

In my opinion, the size of the individual objects isn't the thing you should think about, at least not in the very beginning. Way more important is, how much you want to show, considering the whole screen.
Do you have a fixed window-size? It's no shame to do so, esp. when you're not very experienced. If not, at least try to figure out on what kind of screen you're game is going to be played and if it's more likely to be played in fullscreen or in windowed mode. Done that? Then you hopefully have a somewhat uncertain guess, what the resolution of you're game is going to be.
Now think about how far zoomed out you'd like your camera to be. How many objects of a type should fit next to each other on a single screen? Got a number? Good. If you haven't, check again, whether it's really a good FOV for the game you're trying to create, because any mistakes in the early planning will result in a lot of extra work later on.
Now that you have these basic numbers, just devide the horizontal resolution by the number of objects that can be displayed next to each other and/or do the same vertically. What you get is a very first number about how big your object might be. Don't be afraid to alter it (very few artists will like to work on sprites '37.69 px wide') since every value in here is just a very rough guess, but you shouldn't jump from 30px to 200px or it will, naturally, alter quite a bit about what you had planned for the appearance of the game.

If the result was a very low resolution, how do we verify that you can still get your style across on these small scales? You won't have to do this for every object, but at least one sprite of every object-type is advisable so that you can get a feeling for how to convey your style using such few pixels. First, do a concept-art of the object. It doesn't matter if it's done by hand or high-res on the screen, important is that you're really very satisfied with it. Now do the object in the resolution you've come to. If you're concept has the right perspective, you may scale it down as a reference, but make sure to draw this one complete new. Now compare. Is it still the same visual language? You don't have to show every detail from the concept, that would hardly be possible, but it has to feel just as detailed and has to emit the same style. If it does, it's quite likely that you've found a good resolution. If it doesn't, first of all, try again. If it still won't even come close, there are two possible conclusions: 1. You're simply not good enough of an artist, but someone who is might easily be able to accomplish the task. 2. You cannot decide between macro and micro. You want to show a huge lot of things, but each of them should be as detailed as if it was to cover half the screen. That simply doesn't work, or at least not without a well worked-out zoom-system. Think about your game and decide whether macro or micro is more fitting and more important for your game and work from there.

Hope I could help you with this,

bw,
Tobl

Tobl had a pretty good overview, so I'll just say that it's much nicer to have the sprites themselves at a fixed resolution, rather than scale them down/up in the game to the appropriate size. That way, the artist can adjust for the scale changes themselves and not have to worry about the game smashing their precious pixels together in unpleasant ways.