I need to resize some digital artwork made in PS for print....?

I have some graphics I worked on and posted to tumblr and a couple other social media sites, but now I have to print them, they're originally in 500px by 650px or 500px by 750px. Problem is when I try to print them on regular A4 size paper, they lose all sharpness and quality, the text becomes grainy and the images and accents all have a haze on them. I've worked out how to resize a few, and I have only a couple psd's of them since I made them originally for digital publishing and not print. So does anyone know a way around this? Printing, or resizing the image without losing it's quality or keeping sharpness. I'm still in college learning graphic design, and most of it's self taught for me, so please be kind, I don't know much, this is the first time I'm having to print my work out for show, rather than publish to online sources.