What is the partial molar volume of epsomite (magnesium sulfate heptahydrate)?
As you know, partial molar volume of given substance is the change in solution volume after one mole of this substance is added. When you add $\ce{MgSO4.7H2O}$ to water it dissolves to magnesium cation and sulfate anion but the hydration water simply mixes with the rest of water. So, I suppose that partial molar volume of the magnesium sulfate heptahydrate is the sum of partial molar volume of magnesium sulfate and seven times molar volume of water. If $\widehat{V}_i$ denotes partial molar volume of substance $i$, and $V_i$ denotes molar volume, then: $$\widehat{V}_{\ce{MgSO4.7H2O}}=\widehat{V}_{\ce{MgSO4}}+7V_{\ce{H2O}}.$$
Nevertheless, partial molar volume is function of composition and commonly quoted values are for very low concentration (adding substance to large quantity of water). If you want to obtain solution with higher concentration, finding its exact volume requires knowledge about $\widehat{V}_i$ as a function of composition and such function has to be integrated.