Setting of Width and Image corner

1.How to set the width of photo collage box to 500px width?

A Photonic gallery will take up 100% of the width of its container, so if your container is 500px wide, Photonic will fill it out fully. If you are still interested in forcing it to be 500px wide, you can do so with CSS, but given that your page is a lot wider, it will not look nice. You can force it to be 500px with this in Appearance → Customize → Additional CSS:

.photonic-flickr-stream { max-width: 500px; }

2.How to change the grip of photo to non circle rounded corner?

Didn’t understand your question. Are you saying you want to make the corners of your photos right-angles, not rounded? Photonic creates right-angles – your theme is causing the issue here by changing it to rounded corners. You can override it with custom CSS:

.photonic-flickr-stream img { border-radius: 0 !important; }