3) I'm assuming you want the featured slider images to be a larger size. I'm quite a newbie myself, but I have learned one trick.
In your theme functions (functions.php), use ctrl+f shortcut to find the following:
add_image_size( 'archive-thumbnail', 470, 140, true );
add_image_size( 'single-thumbnail', 470, 260, true );
add_image_size( 'featured-thumbnail', 750, 380, true );
add_image_size( 'slider-nav-thumbnail', 110, 70, true );
Change the figures in bold to the dimensions of the images that you wish to use in your slider. By doing this, you stop your theme automatically resizing images to the default 750x380 when selecting them as a featured image. You must re-upload new images as featured images for them to be cropped at the new dimensions.
You may then need to change the size of the slider, but I am not 100% on whether this is needed - nor how to achieve this - so try the above first and see if it does the trick.
If you meant that you wanted to change the smaller image sizes which are found underneath the slider, change the figures I have placed in italics.
Personally, I used this to extend the slider to full page width (940x397). This was achieved by using the following CSS: