Hi, i am designing my first frame and is a bit unsure how thick my arms should be to avoid breaking and minimize vibrations.
It's a tricopter ment for 10 inch props and will be about 1.3kg in total. And use 3110 900kv motors with a 6s li-ion battery, if this is relevant info.






So far i made the arms 225mm long (from middel of frame to middle of motor), 8mm high and 12mm wide. I belive this should be ok but i have no experience with stress testing and sutch. 
If anyone have any opinions i will be grateful.

From the pioneer of tricopters back in the day, David has made his designs available maybe you can use them for reference. The v4 was designed for similar motor size.
https://rcexplorer.se/product_archive/20...t-archive/

I had a smaller 9" setup with 2212 motors and used carbon fiber tubes. Probably not as stiff/robust as solid CF, but imagine you would save quite a bit of weight and cost, also allow for easy swapping to find the right sizing. Maybe use double-barrels?

i would go for 6mmx6mm or up to 8mmx10mm

rcexplorer as the source for experience on tricopters also came to my mind while reading the question, he might be the best experience source.