Your window/ setting is very pretty. i would add either cafe curtains on rings ( easy open and close) or white plantation shutters.

HereHere’s one shade vs. two shades:

With such a deep sill, I would get two roman or woven. No curtain.

First layer could be cellular whether blackout or plain -- by the street, top pulls down [or pull bottom up like usual].

I like pretty fabric at a window then.

One shade makes the window look more horizontal, two more vertical. How does that concept fit with the rest of the room? I also think about sun. I have some windows that depending on the time of day, the sun just blinds me when I'm in the room. It's a very wide window but the sun sets at one spot. Two shades would allow you to block the sun on one side but not the other, if that is a consideration for you. Maybe not one. What about the mechanism to control them? Will one large one get very heavy and hard to pull up?

If there is no other reason to consider, price is one. Which is less expensive, one large or two small?

Here it is this morning in full sun.

Seeing the pictures of the rest of the room (your other thread), I think Roman blinds/shades would look nice there:

I love formal design so I would go full Dowton Abby on this.

The baseboard heater in your last photo seals the deal. You'll need to keep the treatment within the window recess. Which kind depends on your decorating style., so shop for your furniture look first. Traditional could use cafe curtains, while more modern styles would do better with shades of some kind.

@decoenthusiaste

Our heat is hot water, so long drapes arent an issue here, thank goodness!

I think the shades in the pic from Jinx would look great.

@Jilly

I LOVE the picture you posted!

Is the window ledge wide enough to make it a window seat? Or add pillows at @Jinx did. Her post looks the most versatile for just about any decor. The pillows help to elevate the look of the shades.