What is the largest hydrogen-burning star?

I assume by largest, you mean largest radius.

Well it won't be VV Cep B since this is merely a B-type main sequence star.

O-type main sequence stars are known and these have both larger masses and larger radii on the main sequence (when they are burning hydrogen in their cores).

A selection of the most massive objects can be found in the R136 star forming region in the Large Magellanic Clouds. If you look at this list (though I recommend having a look at the primary literature), you will see that O3V stars are listed. Such objects are also present in our Galaxy, for instance in the supercluster NGC 3603 (Crowther & Dessart 1998).

Such stars have masses of maybe $100 M_{odot}$, luminosities of $2times 10^{6} L_{odot}$ and temperatures of 50,000 K. Using Stefan's law, we can deduce radii of $sim 20 R_{odot}$.

There are suggestions that even more massive main sequence stars have existed in R136 and NGC 3603 (see Crowther et al. 2010), which are now seen as evolved Wolf-Rayet objects, possibly up to $300 M_{odot}$ on the main sequence (though this is a model-dependent extrapolation), and these would have had radii $>20 R_{odot}$.

In the very early universe, population III main sequence stars without metals could have been much more massive and larger.