Start with 7 then double it over and over again.

How far can you get without a calculator?

7
14
28
56
112
224
448
896
1792
3584
7168
14336
28672
57344
114688
229376
458752
917504
1835008
3670016
7340032
14680064
29360128
58720256
117440512
234881024
469762048
939524096
1879048192
3758096384
7516192768
15032385536
30064771072
60129542144