As far as I'm aware, there is no way to manipulate the system crosshair picker, however, Skitch (free!) offers an alternative workflow for partial screenshots. They have a zoomed preview dialog, and the fully extended arms as you desire. Also free in the Mac App Store.
Update 02/03/2020
Kinto - Type in Linux like it's a Mac.
Kinto has now been rewritten in C for Ubuntu/Debian systems using x11. It also uses json config files, making it easier to manage and extend to other applications than just terminals. The app no longer maps to Super in the Terminal apps, it will now properly map to Ctrl+Shift to create the exact same feel as having a Cmd key.
Please checkout the latest release.
https://github.com/rbreaves/kinto
The main change to allow for the Super = Ctrl+Shift change is in this symbols file.
default partial xkb_symbols "mac_levelssym" {
key <LWIN> {
repeat= no,
type= "ONE_LEVEL",
symbols[Group1]= [ Hyper_L ],
actions[group1]=[ SetMods(modifiers=Shift+Control) ]
};
key <RWIN> {
repeat= no,
type= "ONE_LEVEL",
symbols[Group1]= [ Hyper_R ],
actions[group1]=[ SetMods(modifiers=Shift+Control) ]
};
};
You may find Kinto to be useful. It's a project I recently created, after a few years of tackling this problem a few different ways and I just now feel like it's solid enough to share.
The Kinto installer simply uses python to do the initial install, after that it's just a systemd service, bash, xprop, setxkbmap and xkbcomp. As far as I am concerned it is the fastest and most native solution to this problem that you are going to find.
https://github.com/rbreaves/kinto
https://medium.com/@benreaves/kinto-a-mac-inspired-keyboard-mapping-for-linux-58f731817c0
Here's a Gist as well, if you just want to see what is at the heart of it all, it will not alternate your keymap when needed though. The Gist also does not include custom xkb keymap files that setup macOS style cursors/word-wise manipulations that use Cmd and the arrow keys.
https://gist.github.com/rbreaves/f4cf8a991eaeea893999964f5e83eebb
Best Answer
You can set up a custom shortcut, for example a key, for that purpose.
Open System Prefrences, go to Keyboard -> Screen Shots (or navigate to the Screen Shot-options), and double click on the shortcuts you want to change to change them to the keys you use less frequently. If you have a huge keyboard like me, you can for example set the F5 and F6 to be the shortcuts.
Here is a screen shot of the settings window where you set the shortcut (captured with those as well!) The "Copy"-screenshots are the ones with Control+Shift.