The question shouldn't be "what should my mana pool be?", it should be "how am I using the mana pool that I have?".
If you're always going LOM or OOM, you need to fix your mana. There are a few ways you can do this.
1. Unnecessary casting. A misused spell means misused mana. Maybe you're casting a nuke that hits for 500 and costs 250 mana when you have a 450 nuke that costs 100 mana. Also look into how often you're casting. Maybe you keep casting too often and pull agro off the tank. Than you use a heal to quickly heal. Instead you can time your casts farther apart to prevent gainging agro and using the mana to heal yourself.
2. Increase Mana Pool. Increasing your mana pool is easiest by getting +mana gear. Stats will get you some but not as much as pure mana.
3. Mana Preservation. Start looking into mana preservation gear and aa's. This will lower the amount of mana used on spells. This type of gear wasn't easy to obtain for non raiders until DON. You can earn crystals and buy mana pres gear or augs.
4. Mana Regen. The most mana regen you can have worn is +15. There are some aa's that increase it to +20. Some classes also get +3 aa's. This makes a total of +18 or +23 with all aa's. If you raid you will easily max the worn cap. If you don't raid than Mana Regen gear is a little harder to obtain. DON also helped ease this for casual players. You can obtain a +1, +2, and +3 aug. If you plan on buying +mana regen gear in the bazaar it is rather high priced.
5. Mount. You can buy a mount for medding while casting and fighting. The reason I put this last is because of the cost and it's limited benefit. I recomend only getting the 10k mount for casters unless you want the speed and have the money to spare for the faster ones. If you're going to purchase the DOD expansion I recomend buying the boxed version. This includes a free mount. Mounts are nice to regen faster than standing and while on them you don't get sitting agro. They are limited though and can only be used in certain zones (mostly outdoor).
It doesn't matter what your mana pool is. As long as your using it in an efficient manner. You will go LOM and OOM. However if it's a regular occurence and your down time is long, than you need to change something.