As you have found out Lavatera’s are very fast growing shrubs and can grow to a 2 metre height and spread. It flowers profusely from June-September and will always benefit from a regular feed, I suggest a sprinkling of Fish, Blood & Bone on the surface every couple of weeks during the growing season. Sounds like your shrub has been either over watered or under watered so keep an eye on the watering.
With regards to pruning, I suggest to prevent your shrub from getting straggly, cut back hard to within 30cm (12in) of ground level in spring, after all risk of severe frosts has passed. After pruning apply a generous 5-7cm mulch of well-rotted garden compost or manure round the base of the plant. In exposed areas shorten branches by up to one half in autumn to minimise the risk of wind damage.