You can do this with any size, but I am going to show it with the three documents I use the most, 8.5 x 11 portrait and landscape slides as well as a landscape document. Portrait documents are default so I didn't need a shortcut for that.
Step 1: Create a folder in your Google Drive and call it something like Templates or Shortcut Templates. This will give you a place to store these template documents you are about to create so that you remember what they are for.
Step 2: Create a folder in your bookmarks bar. You can do this by right clicking on the bookmarks bar and choosing "Add folder". Name the folder "Templates" or whatever you wish to call it.
Step 4: Change the slide size to be 8.5 x 11 to make a portrait slide set. You can do this by going to File --> Page Setup and choosing Custom. Then change the units to inches and set it to 8.5 x 11.
Step 5: Now that you have your portrait sized slides file, click on the little star to the right of the web address in the address bar to create a bookmark for this page.
Step 6: Change the Folder option to be the Templates folder you created in step 2. Then before you save, click on the "More" button to get more options.
Step 7: Name the bookmark whatever you wish. Then look at the URL (web address). At the end of the URL, there will be something that says "/edit...". It might have some other characters after it, but that is ok. Change the /edit and everything after it to say "/copy" and then click "Save". See the pictures below.
Step 8: Now repeat steps 3-7 to create bookmarks for a Slides file that is landscape and a Google Doc that is landscape.
There you have it! Now whenever you want a Google Slides or Document file that is one of those sizes, you just click on your templates folder on the bookmark bar.
When you click on one of your template shortcut bookmarks, it will prompt you to make a copy of your template.
Now you'll have your already resized document and be ready to go. It took a little bit of set up, but so many clicks are saved each time you do this. Enjoy all your extra seconds. :)