240 x 320 (small phone)

320 x 480 (iPhone)

480 x 640 (small tablet)

768 x 1024 (iPad - Portrait)

1024 x 768 (iPad - Landscape)

This is a free tool to check site responsiveness also known as mobilegeddon or multiscreen and multidevice checker
Check your site here for various sizes and check how it looks. If you found any problem there you can also contact me, I have good experience with Responsive designs.
Note: The content width may not be pixel perfect - I have added 15px to the iframes to cater for the scrollbars