Socket Number Based on Item Size?

I've been trying to get a five socketed item for my sword/board duelist but it seems like all my gear is only big enough to hold a maximum of four sockets. I did read in the wiki that the maximum number of sockets allowed is based on level, aka gear is required to be at least level 50 to have 6 sockets, but that doesn't answer my question on the size of an item and it's socket potential. As a duelist I use a round shield which is smaller than a tower shield. It seems like round shields are only big enough to hold 4 sockets while towers can hold 6. Can somebody please point me to the information I require?

Thanks!

There are no official statements or rigorous data collection on this. But it does seem that item size limits possible sockets - someone reported having a Corroded Blade from Piety (ilvl 70), throwing many Jeweller's Orbs at it, and never getting more than 3 (IIRC) sockets.

However, you DO realize that one-handed items (one-handed weapons, shields) can never have more than 3 sockets, and thus item size is NEVER going to be a practical issue, right? (Corroded Blade and Gnarled Branch are the only two items for which item size matters for sockets, and why on earth would you want a 4S+ Corroded Blade or Gnarled Branch?)

EDIT: Looks like I either read/skimmed too quickly or you edited in specifics after I wrote my post. It seems you did not know this.

One-handed items (one-handed weapons, shields) can never have more than 3 sockets, no matter the ilvl.

Also relevant: Helmets, Gloves, and Boots have max 4 sockets

http://pathofexile.gamepedia.com/Sockets#Sockets

One handed weapons and shields get three sockets. Helms, boots, and gloves get 4 sockets. Bows, 2 handed weapons, and armor get up to 6 sockets. No, you can't get a 6 socketed shield, that would be ridiculously op.

Thanks for the feedback. Looks like the only way I'm going to get my double strike to have life on hit, life leech, extra melee damage, and attack speed is through my body armour.

BTW, any suggestions on a 5th support gem for double strike if I happen to make it to full 6 socketed link?

I run Double Strike with
Blood Magic
Faster Attacks
Multistrike
Melee Physical

As a 6th I would go with either. Life Leech, Melee Splash, Life Gain On Hit, or maybe even another added elemental damage on it. if your weapon has high elemental (Added Elemental Damage), or added fire cold or lightning.