There are many manufacturer's of solar water cylinders and solar thermal storage cylinders.

Many companies will manufacture a cylinder to your exact requirements.  In any case, as long as the cylinder has a good thermal properties and the internal coil surface area can dissipate the heat generated by the solar panels you will be in good shape.

If for example install 10KW of solar panels but the coil in the storage cylinder can only absorb 5KW, the entire system will not work to its full potential.

It is import have a professional design and install your solar thermal storage system.