Since you say you want “help to figure out the Dimensions of the 8 sides” it would appear that it is, indeed, a truncated octagonal pyramidal shape you wish to build (rather than a hexagonal one; you may just have confused the names?). However, we would still need you to confirm this as a first step.
We call the distance between the opposite corners of an octagon (a hexagon too) a Diagonal (though this would be the same length as the diameter of a circle that circumscribed (fitted neatly around the outside of) such shapes).
However, please look the attached diagram. The Diagonal shown of the octagon (circumscribed in red) has been drawn as a blue line and named D. A green line has also been drawn showing the distance across from one side to the opposite side. We might call this the Width of the figure and hence it has been named: W.
As dp says (in post #2 above), you will need to specify whether the dimensions you provide refer to the Diagonals of your shape or the Widths (for both top and bottom).
As dp also states, once these points are clarified, calculating the dimensions of your external surfaces becomes reasonably straightforward but, if you intend to construct this object from 1.8 cm thick wooden sections, then you will also need to know not only the correct mitre angle (67.5°) to ensure sound joints between the sides but both the tops and the bottoms of each side will require to be cut at a calculated angle to ensure that the completed object sits neatly on the floor and presents a flat (horizontal) top edge to the construction. Thus, as was indicated by dp’s post (#2), it is possible (indeed, likely) that you will require more information than you have, ostensibly, asked for and this will not only involve “more work” to provide these but will also require more work to a high degree of precision on your part than simply cutting out 8 trapezoidal pieces of wood!
The diagram (qv) also attempts to illustrate how one of your finished sides might look. I presume you have the ability (tools & skill) to cut your materials to the required dimensions once provided? However, I am curious as to what functionality, if any, this piece of “furniture” is intended to provide or is it meant to be purely aesthetic?
One further caveat you might wish to consider: given the quite thin material you wish to use, if the sides are simply glued together, any load placed on top of the construction will result in a significant proportion of its weight acting horizontally outwards through the joints so the joints would need to be able to withstand that.
Please confirm that your construction is to have eight (equal) sides
and
please state whether your dimensions of 120 & 85 cm refer to the Diagonal (D) or the Width (W) of the base & top of the construction respectively.
Also, as I say, I am very curious as to what is the intended “use” of your construction. Would you care to comment further on that?
