Is there a way to resize Dock in macOS using keyboard shortcuts?
The approach I ended up taking was a variation of @Paolo's solution, and is simple to implement using a tool like BetterTouchTool (as @blizzrdof77 mentions in his original question). 

Instead of preparing two AppleScripts, this single script takes one argument - a numeric value between -1.5 and 1.5 - which determines whether the dock will increase or decrease in size. 

The AppleScript:
Create a new file called "change-dock-size.applescript" with the following content:

on run argv
    tell application "System Events"
        -- Get dock size
        set docksize to dock size of dock preferences
        -- Increase or decrease based on argument version
        set docksize to docksize + (item 1 of argv)
        -- Constrain value to 0.1 -- 1.5
        if docksize &gt; 1.5 then docksize = 1.5
        if docksize &lt; 0.1 then docksize = 0.1
        -- Set dock to the new size
        set dock size of dock preferences to docksize
    end tell
end run



Running It From The Command Line:
You can run this from the command line using osascript - like this: 

# Increase Dock Size
osascript /path/to/script/change-dock-size.applescript 0.01
# Decrease Dock Size
osascript /path/to/script/change-dock-size.applescript -0.01


Adding Keybindings to BetterTouchTool:
To use this in BTT, add two new shortcuts with the "Execute Terminal Command" action, and use the above examples as the commands (I've provided a screenshot of my setup below). I hope this helps!