Simple  tv top with wall paneling..

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why hear ppl say.... i got 5 in my house

normal clean filter, DIY just as easy as any wall unit

for chemical wash, just call a/c man lar, charge also no difference as wall unit, depend on H.p.

For designer, I guess they do have some rules or guide from their school time.

I dont think there is a so call standard size, and since is custom made, that will always depend on your budget, your need and your space, for those size of 8ft x 6ft, i think that size is becoz of budget, usually wood size are limited in 8ft, so if u do shorter like 7ft or 6ft the cut out may need to count in as wastage, then why not just do 8ft(L ), then do 6ft height, mean there will not up to ceiling height, so those wood can cut direct from factory and just fix it to wall then run away, saving labour cost, coz you dont have to box-ing the ceiling or left and right to fit it nicely.

So, if u have budget, u can do biggest as possible, is up to your prefer.

You are the house owner and should know your requirement for TV cabinet. 
Example, my style is dun like many things on TV wall to distract my viewing. Thus, mine just normal cabinet with no wall panel.
For the material wise, different desgn having different price.
To avoid confusion, better provide them a picture with dimension, then can get apple to apple comparison.
Saw simple til complicated TV cabinet before, it made a big different.

bought this at homedec rm 3300, price okay?[attachmentid=3137761]