Resize thumbnails and change their position in home page

Hi!

I have a problem in my home page. I have created a page with two columns, and they host differents categories of portfolio. The thing is that the porfolio thumbnails appear very little (even though I have selected large size in the shortcode). I want to make them larger.
Moreover, as in each line appears two thumbnails, I would like that it appears just one (not a pair).

Thank you for the support!