Is it possible to resize third party icons in wingpanel?
There is a set of extra icons meant for third-party apps that you can use called elementaryPlus that fit well with the default elementary icon theme.
First you'll need to download elementary tweaks (allows for customization of gtk themes and icons and more). Using command line:
sudo apt-get install software-properties-common sudo add-apt-repository ppa:philip.scott/elementary-tweaks sudo apt update && sudo apt upgrade sudo apt install elementary-tweaks
You can then download elementaryPlus icon set with command line:
sudo add-apt-repository ppa:cybre/elementaryplus sudo apt update && sudo apt upgrade sudo apt-get install elementaryplus
Go to System Settings > Tweaks > Appearance and set Icons to the 'elementaryPlus' option.
I'm not completely sure if elementaryPlus has an icon that matches the application you are using, but it's worth a try. Perhaps you could also take a look at various other icon themes such as numix, paper, etc.