@liliana.duarte you can choose the size of the pics on the gif there, too, you don't need to resize them, you will just have to fix the settings.
You could also set it to slow or fast intervals - all you have to do is to drag the set of pictures, wait till they all upload, rearrange the pictures because they tend to get rambled, press continue and it'll ask you how you want your gif to be made and then adjust everything you want to adjust on the settings provided there. It's very handy. Good luck