How to signal that a last letter vowel is long or short

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.,