Do not worry You do not need to hire a designer to decide for you.
Just follow these simple steps:
Because you know where you want to hang the painting, take the dimensions of your available space.
Give yourself a little wiggle room so you have more freedom in your choice.
Go for the style and colors that you like, do not try to match your existing sofa, rug or window treatment . It will all come together just because that's the colors you are naturally drawn to.
Do not buy too big for your space. You do not want the piece to be cramped .
Do not buy to small and let think you could not afford more.
Buying art IS a an investment. If you have a precise space you want to embellish with art and can't quite yet afford it. You might want to wait a little bit and put some money aside for that purpose. You might be happier in the long term if it helps you choosing the perfect artwork.
Respect the volumes in your room.
If everything is horizontal and you have a good ceiling height you want to choose a vertical shape,
If you do not have too much ceiling height then go for a smaller horizontal shape.
Trust your eye.
And then you can buy an artwork just because you fell in love with it. And I'm sure you are going to find a place for it...