This question is of interest because there are terrestrial organisms, such as bacterial and fungal spores, and even complex multicellular organisms, such as tardigrades, which can survive deep space conditions indefinitely. Such organisms, if cooled to cryogenic temperatures (e.e., below -100 degrees C, exhibit profound radiation resistance and a practical arrest of all biochemical activity, potentially allowing for millions or even billions of years of survival. If such organisms were ejected into space via meteorite impact or ice volcanism, they might successfully traverse interplanetary and, more problematically, interstellar distances.
The largest viruses measure 0.5 μm. Bacterium typically measure up to 2.0 μm but their spores are a bit smaller.
Particles of 0.6 – 60 μm radius can remain cold enough to preserve organic matter during atmospheric entry to planets
For anything where conventional physics is in play getting from orbital velocity of at least 7 kilometers per second to approximately zero on earths surface involves dissipating 30 kJ of energy per gram. This is easily enough to heat that gram from below zero past boiling. This ratio stays fixed as mass reduces, at least to the point where we start talking about individual atoms and molecules.
In addition individual cells or clusters of cells are unlikely to handle individual high energy strikes in the upper atmosphere well.
So we are unlikely to see a single organism make a journey across space and re-entry alone unless it uses exotic chemistry.
Panspermia theory generally assumes cells hidden from the extremes of space and re-entry within larger rocks, small sections of which can sheltered by ablation and insulation of the parent body. So the concern from extra terrestrial contamination probably increases as object size goes up, at least until the body is large enough to make substantial craters, though even there smaller chunks might be shed during re-entry in a manner that is not immediately lethal.
This means that the technical upper limit for 'delivering a payload kept below 40 degrees C' is probably when the impactor is so large that even the chunks trailing behind the main impactor are landing in lava or boiling ocean.
There's an interesting passage in the Smithsonian Scientific Series "Minerals from Earth and Sky - THE STORY OF METEORITES" at page 22:
Of all the known iron meteorites, but seventeen were seen to fall, and of these only that of Mazapil, Mexico, need be given in detail. The account is that of one Eulojio Mijares, a ranchman living in Mazapil. This fall is of special interest, having taken place during one of the periodic meteor displays.
We returned after a little and found in the hole a hot stone, which we could barely handle, and which on the next day looked like a piece of iron.
There's no exact info on how much time passed between touchdown and handling, nor on the exact temperature, but it seems that we're not talking about molten iron lumps here.