There is a lot of energy and movement in this painting. And wonderful color harmony. Everything seems to be painted loose, but not sloppy. The doll adds a lively interest to an otherwise calm scene.

Your sizes can be whatever you wish. As goldensun points out, various sizes can create contrast and interest in a composition. But this can also be done by varying saturation of color, using opposite values, using contrasting shapes or textures, using sharp crisp versus soft blended strokes… One can use some of these things to make contrast in a painting You have a lot of earthy and warm colors in your painting, which makes harmony. But then you have the dolls face, which is cooler whites and blacks, and so this creates contrast and interest enough.

This is a superb still life. As long as one of the two objects is different in size, shape, texture, etc., it should work fine. You’ve done that here.

I like your colours!!

I think it has to do with how attention works; if they’re the same size they’ll fight for the viewer’s attention. Personally, I have not seen the vase until I read your post and looked again (also probably because its colour blends with the background). I think if the painting has more objects and the two objects with the similar size are place somewhat apart, there would be no problem at all.