Oh, that makes sense. Smart thinking! They look big enough you can probably get your tubes, small tool kit, mini pump, and a couple 1L bottles of fuel in there. If you put your U lock on the seatpost (mine mounts to seatpost, U lock hangs down from there), you will have alittle more room, and a handy weapon against evil dogs.
How long are the cranks? 175? That is the standard. 180 are rare except for BMX and 29er singlespeed, 170 is common on road and cheap mtn bikes. 165 is common for Junior riders, women, and people who really like pedaling fast and shifting a lot. I prefer 180 on anything.