How many pages is one to show the congregation during Hagbah?
One is to show [up to] three columns of the Sefer Torah to the congregation upon performing Hagbah. Possibly, one is to show exactly three columns [and not less or more]. Some Poskim however rule that it is all dependent on the strength one has, and if he is able to hold the Sefer Torah when it is opened many pages [and hence, if one is unable to open it three columns, he can open it less]. [Practically, one is to try to open at least three columns of the Sefer Torah, and if he is able to, many are accustomed to open even more.]
 Miseches Sofrim 14:14, and 3:5; brought in M”B ibid
 M”A 134:3 in name of Chidushei Aguda; Miseches Sofrim 14:14 see also 3:5 that one is to open the Sefer Torah three columns; Elya Raba 134:4; Kitzur Shlah; Kitzur SHU”A 23:25; M”B 134:8; See Igros Kodesh 18:427
 M”A ibid; Implication of Kitzur SHU”A ibid
 M”B 134:8
 So I heard from Rav Groner Shlita