Sol-gel processing provides a useful route to novel metastable materials such as molecular hybrids of silica and either transition metal oxides or organic components. Because the properties of these materials depend critically on how the components are combined, we present calculations of the size and structure of silica building blocks that may be prepared as precursors to nanocomposites. By fitting existing silicon-29 NMR data, we find kinetic parameters applicable for acid catalyzed hydrolytic polycondensation of tetraethoxysilane and tetramethoxysilane precursors. In a wide range of conditions, the local connectivity of silicon sites evolves in approximately the same way with respect to the siloxane bond conversion. Using dynamic Monte Carlo simulations, including nearest-neighbor effects and cyclization, we calculate the molecular weight distribution of silica as a function of conversion, in reasonable agreement with available experiments. At low siloxane bond conversions (alpha less than or equal to 0.6) the mass weighted degree of polymerization DPw is less than 10. When enough water for alkoxide hydrolysis is available, gel precursors containing 8-20 silicon sites form quickly up to conversion alpha = 0.65-0.75 and then slowly react together until gelling at alpha = 0.82. The calculated distributions provide a quantitative road map for forming composite materials with well-defined silica blocks, where the siloxane bond conversion can be used as an indicator of the progress of structure development.