
I don't get why this limit exists, let alone why the limit is so small. Fortunately, I have found a way to bypass this using javascript.
You need to make sure javascript is enabled/allowed on duelingbook, obviously, and when enabling javascript on duelingbook, you may need to refresh/reload the page for the changes to take effect.
Create a javascript bookmark. Name it whatever you want. For this demonstration, I will be naming my bookmark "deck namer".

In the box that says "URL," copy and paste this line of code:
javascript:document.getElementById("input").getElementsByClassName("input_txt")[0].setAttribute("maxlength", "300");
Click "Save" to save your bookmark. Note that on other browsers such as Mozilla Firefox, the URL box may instead be referred to as a "Location" box or an "Address" box, and the "Save" button may instead be referred to as an "Add" button.


After you save your bookmark, go to duelingbook's deck constructor, and select either the option to create a new deck or the option to rename an already-existing deck in order to bring up the menu where you have to input a deck name.


Click on your newly created javascript bookmark. Now you should be able to enter a name that is as long as you want, without being limited to 25 characters for your deck name.


That's how you bypass the character limit for deck names in the deck constructor.