The hard working Jan Lahmeyer compiled a great many tallies or estimates of historical populations [as of 1/5/2019 my emails to him are being returned – does anyone know if he is alive?]. At http://www.populstat.info/Asia/indonesc.htm for Indonesia he gives 28,992,000 as the population for 1870 and 35,579,000 as the population for 1890. The current population of Indonesia of 262,787,403 (July 2018 estimate) was influenced by the Indian Ocean earthquake and tsunami of December 26, 2004 (an estimated 130,000 dead – about one half of the total deaths – were in the Banda Aceh area) as well as territorial adjustments.
As with many straits, even those where one country owns both sides of the strait, assessing regional populations is not straightforward. Second order political subdivisions such as states and provinces change over time. In the case of the Sunda Strait, the islands of Sumatra and Java are so large (180,000 and 55,000 square miles, respectively) and oriented more or less perpendicular to the strait that really only the one province of Lampung on Sumatra and the one province of Banten on Java will be likely to bear the direct impact of a tsunami in the strait.
Lampung province currently has a population of about 8,300,000. Its capital, Bandar Lampung, has a population of about 1,200,000.
Banten province currently has a population of 12,500,000. There is a fair amount of dispute about whether Tangerang is actually an independent city or a suburb of Jakarta. Tangerang has a population of about 1,800,000, while Serang, the provincial capital, has a population of about 600,000.