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echo `cat /sys/class/block/sda2/size`*512 | bc

or if you use bash, you don't even need to call bc

echo $((512*$(cat /sys/class/block/sda2/size)))

gives size in byte

echo "`cat /sys/class/block/sda2/size`*512" | bc

or if you use bash or any other POSIX-like shell whose arithmetic operators work with 64bit integers, you don't even need to call bc

echo "$((512*$(cat /sys/class/block/sda2/size)))"

gives the size in byte.

The call to cat and fork (except for bash) can be optimised away with bash, ksh93 and zsh with:

echo "$((512*$(</sys/class/block/sda2/size)))"

echo `cat /sys/class/block/sda2/size`*512 | bc

gives size in byte

echo `cat /sys/class/block/sda2/size`*512 | bc

or if you use bash, you don't even need to call bc

echo $((512*$(cat /sys/class/block/sda2/size)))

gives size in byte

echo ``cat /sys/class/block/sda2/size*512 | bc`

gives size in byte

echo `cat /sys/class/block/sda2/size`*512 | bc

gives size in byte