This will be my first tri though I've already built a quad. I want to make a medium-sized tri with good stability. I've read to make a tri stable u must make the tri's arm length double a quad's arm for the same stability. Please help me choose the arm length.
I'll use these parts:
Motor- 2212/13 1000KV Brushless Outrunner Motor With Mount
http://www.quadkopters.com/product/221213-1000kv-brushless-outrunner-motor-with-mount/
ESC- 20 Amp Simon Series ESC
http://www.sp4mm3r.com/speed-controller/simon-series-25a-esc-2.html
FC- Apm 2.6
And
Tail Servo- Avionic Metal Gear 9g Servo (AV9DMG) - Digital
http://rcbazaar.com/products/799-avionic-metal-gear-9g-servo-av9dmg-digital.aspx

Primary factor deciding arm length is size of the prop, then the purpose as for FPV you will need longer arms so that the props can be out of camera view.Or because of space requirements or frame size.

It is the first time I heard about "tri's arm length double a quad's arm for the same stability". Stability would largely depend on your PID's and RC rate or expo etc. Longer arms will help in FPV or Aerial Photography,because movement will be less at the center than at the end of the arms. Similar to the funda behind a prop turning on a central point, wherein surface closer to the hub would be travelling slower than the tips of the prop.

Forgot to mention it I'll be using a 10 x 4.5 prop and no FPV but I would use it for aerial photography. I'll be using carbon fibre for the arms.

I need it to be stable I won't have any problem even if it is large in size.

Don't remember it clearly where I read it, maybe read it a week or two ago, maybe in an rcgroups.com topic.

See stability of your copter would depend on your FC and how you set it up.Length of arms wont be a major factor affecting stability if they are strong enough and as per dimensions etc.

For stability in FPV/AP use a gimbal (servo/brushless).

350/400mm  MtoM is good enough for a 10" Tri. To get a perfect prop free view, design your frame accordingly.
A t-copter design would be easy and great for FPV. Attaching design specifications.

I'll be making a Y-copter so will the same specs be suitable for a Y-copter.

No, that will have different dimensions. Simple thing to follow while designing and selecting arm length is that all the motors will be spaced equally from each other in tri whatever the size may be

If you have a circular center plate, then length of arms would be equal, if not they will vary as per your frame design. Its a simple geometric design, you should be able to calculate arm lengths yourself.