TL;DR-- I think even under ideal circumstances, a field strength of about $0.4$ Tesla would be the limit.
So first off, ArtificialSoul is probably correct that no one here is going to be able to give you anything resembling a super accurate answer. But like most problems in fluid dynamics, If I rapidly wave my hands fast enough I can give an extreme estimate that should give you an idea of the order of magnitude that's possible.
Now, the actual planetary dynamo of the Earth is due to the coriolis force acting on convection currents of molten iron, twisting them into spiral flows. Then, a complex feedback mechanism between current carried by these flows, previously created magnetic fields, and resistive dissipation combine to produce a somewhat stable magnetic field. This is an absolute nightmare to model. So, instead, I'm instead going to approximate these flows as a current density
$$\mathbf{J} = J_0s\hat{\phi}$$
Where we are in cylindrical coordinates and $s$ is the coordinate telling you how far you are from the z axis. The current will follow this form until the radius $R$, at which point it will become $0$. $R$ in this case is the radius of the zone where the iron is molten-- the outer core in the case of earth. I am going to neglect the fact that the inner core is solid, because less work = good.
The reason I chose the form I did for the current density is because it's the same as the current density of a charged sphere azimuthally rotating at some angular velocity. Again, the real picture is much more complex, but this will give us a good upper bound because it assumes all the flows of molten metal are working together to generate a magnetic field, whereas in reality, different regions will be counteracting each other and just making a general mathematical mess.
Now, we will use Biot-Savart to calculate the magnetic field along the z axis, because I would assume that's where the magnetic field would be strongest. More Importantly, however, it's much simpler. Now, Biot-Savart states
$$\mathbf{B} = \frac{\mu_0}{4\pi}\iiint_{current}\frac{\mathbf{J}\times\mathbf{r'}}{(r')^3}d\tau'$$
I've made one more simplification, which is that I'm ignoring magnetization of the molten iron and I'm simply using the permeability of free space. This is reasonably accurate though since molten iron is well above the curie temperature and thus doesn't act very magnetic.
Plugging in all our nasty expressions in with the proper coordinates and simplifying considerably, we end up with the following expression (as long as $z>R$):
$$\mathbf{B}(z\hat{z})=\hat{z}\frac{J_0\mu_0R^2}{2}\int_{-1}^{1}du \bigg[ \frac{1-u^2+2(\gamma-u)^2}{\sqrt{1-2u\gamma+\gamma^2}}-2(\gamma-u)\bigg]$$
where $$\gamma = \frac{z}{R}$$
This seems really awful, but it's actually not too bad-- the integral is simply a function of $z/R$ that decreases like $1/z^3$ as you move far away. For clarification $z$ is the coordinate along the z axis where $z=0$ at the center of the planet. From this point on, $z$ will refer to this coordinate at the surface of the planet, since that is where the magnetic field is strongest.
The key points to take away from this is that $\mathbf{B}$ grows with increasing $J_0$ and $R$ given fixed $\gamma$, and shrinks with increasing $\gamma$. So, to find an upper bound for magnetic field strength, we want $J_0$ and $R$ to be as big as possible, and $z/R = 1$. I'm simply going to reverse engineer $J_0$ from Earth's magnetic field to get a rough order of magnitude estimate, which produces the result $J_0 = 1.4\times10^{-10} A/m^3$ (as always, this is a very rough estimate).
As for $R$, the biggest planet thus far discovered has a radius of about $1.25\times 10^8 m$, so we will use that value. Finally, the integral attains a maximum of $0.2666$ when $\gamma = 1$. Note that this would imply the entire planet is molten and carrying a current-- for planets that have a solid layer, $\gamma$ would be greater than one.
Putting all these values together, we get an upper bound of
$$|\mathbf{B}| \leq 0.38 T$$
at the surface of the planet. This is assuming $J_0$ is roughly the same for any planet, which likely isn't true-- faster rotation would imply higher $J_0$, and complex feedback mechanisms between the magnetic field and the convection currents could cause significant deviation from this value. But regardless, this should give you decent idea of the most extreme magnetic fields a planet can have that retain a shred of scientific plausibility.
I don't have time right now to add more detail of my calculations, but if you're interested I can probably edit them in later. Hopefully this helps!
