I saw in one heavy looking building main structure of the ceiling made of raw tree trunks. But it was pretty small span. What would be the diameter of the raw trunk for 20m span between walls? Or what are usual maximum spans for tree trunks without any modifications?
What do your load calculations say? What kind of wood is it? When you say ‘raw tree trunk’ do you mean raw as in not cut into a nominal/typical size or raw as in not dried yet? Are these going to be structural or decorative? What does your local AHJ have to say about it? They might have codes in place that cover this.
Even with all those questions answered, short of this being purely decorative, I think you’ll need to discuss it with a local engineer.
Also, here’s the first website I saw covering this kind of thing. Contacting them might point you in the right direction.
Depends on the species of tree, and more specific conditions such as the straightness of the trunk. Bends and twists in the trunk, the size and position of limbs make a difference too. For very long beams there are few trees available for long straight lengths of wood and a beam of that length requires a very heavy beam so a very tall straight tree with long consistent grain has to be found. The value of a tree like that is very high when cut up for maximum yield which greatly increases the price of an intact beam of that size.
Such long beams are rarely necessary or used. Timber frame techniques are used to create trusses and join wood to span wide unsupported spaces. Laminated beans are widely used also. They are generally made from softwoods like pine and related species.
I’m going to go out on a limb and say you are not going to span over 60 feet with a tree trunk unless it supports no load and you can withstand some sagging.
ISWYDT
20 m is a huge span for wood framing. You will often see an 8m engineered LVL beam used to support a room above a two car garage. Even that span will be 2 or 3 24" / .6m deep LVLs laminated together. That is the max I’ve seen in residential construction.
LVLs are essentially plywood, typically 1 3/4" thick. They will be much stronger than any timber of a similar mass.
I think the manufacturing process allows them to be made almost any length, but the biggest I have seen them is 40’ / 12m and those were always cut to shorter lengths. Transporting anything longer than that is going to be challenging.
Even a steel ‘beam’ of 20m is likely to be of engineered truss structure rather than simply extruded “I” or box beam.
I depends upon the species of tree. A lodgepole pine is going to be longer and straighter than a swamp spruce. A second growth douglas fir will be thiner than an old growth. What kind of log do you want?
There are still a few sawmills that can cut you an 80 foot beam and be structually sound. If you just want the beam for looks, it doesn’t really matter.
This is what I love about this Board.
A barely understandable OP by a brand new poster, but several well thought out and intelligent responses plus one slightly considered pun 
I volunteer in a old windmill, (1876 era) restored recently after a fire.
We used sugar gum ( Eucalyptus cladocalyx ) for the main beams that need to support around 3 tons of millstones at around 400mm x 400mm.
Distance is not 20m though, it is about 10m
There is a much bigger (but decrepit) mill nearby with probably around 20m needed at the same level (supporting the stones) and that is all English oak, probably 500mm x 500mm. That mill would have had at least 2 sets of stones, so the beams supporting at least 7 to 8 metric tons, plus some floorboards.
The beams are all square-cut heart wood, so I am not answering your diameter question, but this might give you an idea.
The rule of thumb for a beam is that the depth should be between 10% and 12% of the length. That means that a 20 metre span would require a two metre deep beam at least.
True, though I suppose one might build a very large building on site in a forest (for use as a park inn, or the like, for which a rustic, unfinished appearance might also be desirable).
And now I’m wondering what the longest horizontal span is of any tree branch. Though of course that’d be an even harder problem, since it’d be cantilevered, not supported on both ends (and maybe even by pillars in the middle) like a roof beam would be.