Create custom image sizes & increase font size

Thank you for answering my query about the standard size of images for Adventurous pro. The image size should be 800 x 324 but I find this too elongated and it means there are lots of images which I can't use because they lose their important features at this size. Is there a way I can create (higher) images for posts which are not complying with that size guide, without causing any damage to the site speed or anything else for that matter? Are the Gallery Images also supposed to be 800x324? I want to use a more square type image... I also want to increase the font size throughout the whole site. Thank you for your help.