As a Game Master I make up a lot of names for locations, objects, etc.
I've always assumed you signalled it by placing a ´ over the last letter (like the City of Rohvanná), but recently I was told it wasn't like that and I've been unable to find a simple answer by googling, and this place seemed like it might be the right crowd to ask. ^^
English doesn't have long and short vowels. it has tense vowels (often realised as diphthongs) such as the vowels in "cape", ""ripe" and "cope"; and lax vowels, as in "cap", "rip", and "cop". These pairs almost always differ in quality rather than in length (there may be a difference in length too, but we don't distinguish length without a change of quality).
I think what you actually mean is that you want the final vowel to be a full /a/ rather than the reduced vowel /ə/ at the beginning of "about". I think the most common way to signal that is adding -h, to get "Rohvannah". It's not foolproof - names that people are familiar with such as Hannah tend to get the final vowel reduced. But it's probably the best you can do. Your idea of an accent is also possible - a macron ā may be a better choice than an acute á, as it is used only for length and not for stress.
Another alternative in some parts of the Anglosphere is -ar. In non-Rhotic regions such as most of England and Wales, Australia, New Zealand, and a few parts of the US, this is pronounced /a:/, so ma and mar sound the same. However, this won't work if any of your audience are in the rest of N America.,
As a Game Master I make up a lot of names for locations, objects, etc. ...
As others have said, English orthography just doesn't provide a way to unambiguously indicate how words are pronounced.
What's often done in educational materials is to provide a phonetic transcription of a word when it is first introduced, for example,
Your adventure starts in the city of Rovannah (Roh-VAN-ah), ...
If your readers are more sophisticated about such things you could use IPA instead of the simplified spelling I've used in the example.
