Adjust excerpt length

Hi
my website it wonderstruckfawns.com and down the bottom under the recent posts widget it displays a portion of the post. i think its about 50 words at the moment. is there a way to adjust this to only display about 20 words?

Also, once that is resolved i have another issue regarding mobile use. I dont like the way my website looks on a mobile. because i have customised it so much the front page is really broken on a smaller screen. is there a way to turn off this mobile version of the site so that when it is viewed from a smaller device it remains the full website version?

Thanks, Paige.