Mars density is less than 1% of earth's.
This is what earth looks like from 30 000 meters high (where air is about as dense as Mars)
It's going to be too heavy
Using this book as a reference, we can see that to lift 1T to 30 000m we need a 10T balloon (accounting for balloon and lifting gazes).
Of course you'll need more since you'll want to produce positive lift before touching the ground.
An empty (without payload and fuel) Ariane 5 is 50T.
To lift it to 30 000m, the blimp + rocket would be already 500T!
Now that you are at 30 000m, you need to accelerate the whole 500T by 13 500km/h just to reach mars (And I'm not even counting any deceleration).
In order to do that, you would need an incredible amount of fuel, that would in turn increase the mass of your blimp, making the whole thing unsustainable due to tyranny of the rocket equation.
To go to space, you don't need to get really high, you only need to get really fast.
It would not survive reentry
Assuming that you succeeded to sent your blimp to mars (no, you did'nt), you are moving at a an incredibly high speed.
As a reference, the heat shield for the mars exploration rover was 78kg for a 1T payload. To survive the aerodynamic forces (>15G for curiosity), you would need a really strong structure, which can only translate to more mass, which translate to even more mass when the blimp and rocket equations are factored.
It is impractical because
- The blimp itself would be much too heavy
- Mars atmosphere is much too thin
- It offers no advantage compare to a rocket
And that's using very optimistic numbers and discounting many factors who would, by themselves make it fail.
When mass is a premium, you don't want to carry around a heavy blimp.
In orbit, your mass is still the same as it was on earth, and a the blimp that initially lifted you up of the atmosphere is actually heavy.
How big would be a blimp if you wanted to lift at 1atm ?
Since your title question seems to be slightly different, let's try to answer it.
The Hindenburg could lift 10 000kg for a volume of 200 000m3.
A falcon 9 fully fuelled is 750 000kg.
Therefor you need 200000/10000*750000 = 15 000 000 m3 to lift the rocket.
the cruise altitude was 200m, so don't expect going much higher than that.