ID is just a way for you to reference it uniquely. For example if you wanted to apply some css to just that specific input tag.
Size is the width of the input tag.
Name is like giving it a variable. This is how you access the value after submitting the form.
Don't be scared to change things like size and name. It will be easy for you to see how the size tag works.
Also if you do a search on google for <input> tutorial you should get some tutorials that could help.