Troll physics, jumping upwards just before crashing ?

Yes, the size matters. Let's turn this horizontal for a moment. A small child on ice skates pushes off of a large adult on ice skates. The child goes flying. An in reverse, if the adult pushes off of a small child, the adult barely moves.

Going vertical again, pushing off of a feather or a large concrete block matters. But let's assume the block is very massive.

It still won't save you. Yes, you will slow down slightly, but it won't be enough to save you.

Here's how you can calculate it. How high can you jump? The world record for vertical jump from standstill is about a meter. You can calculate that acceleration. It's nothing compared to 9.8 m/s^2.

Or, you can not calculate and just do a rough estimate. The terminal velocity for a human (the speed at which air resistance balances the acceleration of gravity) is about 56 meters per second. The world record jump is one meter. Not going to help.

What would happen? You'd go splat.

Would you slow down your falling speed? Negligibly.

Does it matter the size of an object you are pushing away from? In terms of what, survivability? If it's something that isn't itself going to slow your fall like a parachute, no.