Surely this should be Here is a really large mushroom, right?
Same, pretty sure that should be right:
Täällä - here
On - is
Tosi - really/very
Suuri - large
Sieni - mushroom
"Here is ..." in English implies that you are offering something. There is no such implication in Finnish.
over here is a very big mushroom was not accepted
"over here is a (really/very) (large/big) mushroom" is a correct translation; "here is a really large mushroom" is better (verified)
The word selection allows for multiple answers, unfortunately