Sidebar size & header image

Hello,

Few things I would love for to have some help with 🙂

– I would like to make the right sidebar about half the size (if not less) it is now – without using 3 columns. it currently takes up almost half of the blog area and this is way too big

– I want my header to just include an image ,I have already removed social icons and know how to upload an image bigger than that recommended, but am having trouble with making this image in the centre of the header, it goes off to the left slightly…how can i do this?