using objdump -d , i get
80483b7: e9 73 7c fb f7 jmp 2f <_init-0x8048265>
80483bc: 5e pop %esi
80483bd: 89 76 08 mov %esi,0x8(%esi)
does this mean that for example the size of jmp instruction is 5 bytes ?
Yes, however, you can get jumps of other sizes, so always check first with objdump and consult the intel instruction manuals.