The final size of a heart shape that can be cut from a trillion
depends on a few different things. Most important is the geometry
and the symmetry of the trillion, especially the depth.
There are many dozens of ways to cut a trillion, some shallow, some
not. Also, I frequently see very non-symmetrical stones, with poor
facet meets, uneven girdles and off-center culets, mostly cut for
weight retention at the expense of brilliance.
To cut a proper heart of a chosen geometry means that all those
things must be corrected. They will cost size and weight.
In the end, without seeing the piece it is nearly impossible to make
an accurate estimate of what can be done. Keep in mind that
maintaining maximum diameter will usually be at the expense of
beauty. So, do you want a beautiful smaller stone or one slightly
larger, but lumpy stone?