Let's do Some Physics!
So, you want to use magic to push things around, eh? And the magic is only as "strong" as you are?
Well, let's get some things straight.
First, we're looking at the power a human can output. That's not force, not energy, but how much energy you can put out over the course of a period of time. In theory, a person can move a rock of any size, it may just take them longer than they can live. (Other forces may prevent them from making progress, but let's get back on topic).
Anyways, I'm assuming the magician in question can just transfer a particular amount of energy (as defined by the power they produce) to some other object without losses. This would mean a magician could direct their heat energy to an object kinetic or potential energy, without any losses. I'm also assuming that magicians must continue to input energy to systems they're affecting or else the magic effects immediately cease, and the objects in question behave as normal. (This prevents floating objects from floating indefinitely and requires more work from your magic users.)
Maximum Power Output of a Human
In terms of body heat, a human puts out anywhere from 75 W to 870 W from body temperature alone. (much more than a lightbulb!). That means a wizard has around 800 W to fool around with from his body heat alone! Obviously, a wizard would need to elevate their body temperature before casting spells to take full advantage of that 800 W.
Humans are more than just heat sinks. We can put out around 400 W of mechanical power (for around an hour) if we're well-trained. Less well trained individuals go for around 50-150 W during the same course, though, so the individual's condition matters.
Wait, what about burst strength? That's when you focus your efforts into one task, and you focus as much power as you can into one push or lift. That sounds like a question weightlifters can answer to me! Take a look at the wikipedia page for weightlifting records. It looks like a world-class weightlifter (of weight category ~77 kg) can do the clean & jerk event with about 200 kg! Assuming this action is done it in about 2 seconds, and you are the same height as Oleg Perepetchenov, that's (200 kg * 9.8 m/s^2 * 1.64m / 2s) a whopping 1,597.4 W!
As a favorite thought exercise has reminded me, you technically don't do any work holding a thing up. However, if the additional potential energy the object gets from being higher comes from the person, then yes, the magician will still loose energy. (The added potential energy is $mgh$, where $m$ is the mass of the thing they're holding up, $g$ is acceleration due to gravity, and $h$ is how much higher the object is. If you do that over time, then you get power, measured in watts!) So I'm running with that assumption for holding things up.
What Can I Do With How Many Watts?
For 75 W, you can:
- Lift a household cat (a little on the fat side) above your head in 1s (I'm not responsible for where the cat goes after that.)
- Throw a stool
- Toss a baseball at at 70 mph (not enough to be in the major leagues, but a respectable pitch.)
- Hold that same domestic cat you threw up to head height for 1 second. (If you're putting out 75 W constantly, then the cat will stay up!)
For 800 W, you can:
- Raise a house cat 17.5m (about 57 ft, or 60 yards) in 1s (that's pretty high!)
- keep a house cat at ~13,000 m above its normal height (until you run out of energy).
- Hold a 50 kg (110 lbs.) weight above your head (until you run out of energy).
- Throw a baseball at 105m/s or ~234 mph.
For 1,600 W, you can:
- Throw a baseball at 150 m/s, or ~335 mph (or 0.43 Mach, with 1.0 Mach being the speed of sound)
- Lift 200 kg above your head (that's enough for 2 large men!)
- Raise a house cat 50,000 m in 1 second. (the boundary for outer space technically starts around 100km, or 100,000 m)
Obviously, if you did any of these things slower, or for less time, you would use less power (less watts).
Why am I picking on house cats? I just chose something familiar. Please don't use this magic on cats!