Many times when you read a recipe it will specify the type of cut in the instructions. Without a picture, you may not know how big to cut those onions or carrots. Here is a quick reference below!
Cutting your vegetables approximately the same size will help them cook more evenly. Ever mashed some potatoes and there were pieces that had not cooked enough? This may be because you cut many different sized chunks of potatoes and when you tested them for doneness the fork pierced one cube perfectly, but other larger pieces were not quite cooked... Size matters! Do your best to improve your knife skills and your dishes will improve too!