How do I decide on a continent size?
So I'm working on a map for a new world I'm creating, and I have no idea how big to make the main continent. My world has some other ones as well, but I'm not focusing on them (at least for the time being).
I have a few important landmarks and cities that I have decided the distances between, and I have a scale (I've been sketching my map by hand, so my scale is 10 miles = 1cm), but I have no idea what size would be reasonable for the continent.
I also feel like a lot hinges on this, because the people in this world use horses as their main mode of transportation, and so I haven't been able to decide if this is a continentalized place, which holds me back on developing the inter-kingdom political relations and a bunch of other stuff.
Are there any tips or,,, like guidelines or something that I can use to help me decide on a general continent size (like how wide the widest point is or how long it is)??
ETA: I'm not even sure if I should start on a planetary scale first and determine how big my planet is/what water:land I want/etc, or if I'm being too unrealistic re: my level of ability and where I should be starting.