Triathlon - advice needed on sizing aerobars

i'm going to be adding some aerobars to my road bike for some TTs this
spring, and this will be my first time using aero bars. looking at the different
choices out there in clip-on bars, i see that there are several different
lengths ranging from ~150mm out to ~300mm. is there a good rule of
thumb regarding the size/length needed?? it would seem to me that the length
would be dictated by my position on the bike and the length of my forearms.
my forearms measure ~350mm from the tip of my elbow (when bent at 90 deg)
to the middle of my palm.

thanks in advance for any help.

It's all about overall bike fit. I would recommend getting a bike fit by someone who has experience of fitting cyclists in the aero position. If you already had a bike fit - that positon changes after adding aero bars - so you have to refit.
Even after the fit it is not uncommon to tweak your position several more times. I must have had cut my length on my HED aero bars 4 times - 1/4" at a time after my bike fit. Make sure you ask the bike fitter ahead of time about his policy for tweaking - Most will do it at no charge - BUT ASK.
Do it conservatively - you can't add material back after you cut it. I have also changed my forearm pads to something more comfortable. As well as changing my height of my pads lower and back to my orginal position - again tweaking it after riding and racing. In the end you will have a good positon that works for you with a good bike fit which can only come from time in the saddle. I do a weekly TT series in spring (8 races) and it allows me to try out new things prior to a big triathlon.

Another point I wanted to make is it has alot to do with the distance of your TT's or tirathlons - I have 2-Tri bikes. Bike #1 is set-up for short races (more aggressive/lower more forward) knowing that it may be a TT only or running 10k up to a 1/2 marathon). Bike #2 is setup for Ironman Distance events (comfort is key here knowing I need to run a marathon after my 112 on the bike). If you are doing a TT only you won't care about saving your running muscles, so your ride position can access those for power as well.

Some, perhaps most, aerobars are adjustable. Right now I have a set of profile-design AirStryke bars on my bike. After I put them on I tossed the right size allen wrench into my pocket and went for a ride. After riding for a while I stopped and started making small adjustments. Before long I had them adjusted for the most comfortable position for me. Don't be afraid to experiment with different adjustments until you find the position that's best for you.