Block Size
The block size determines how many bytes radare2 commands will process when not given an explicit size argument. You can temporarily change the block size by specifying a numeric argument to the print commands. For example px 20.
[0x00000000]> b?
Usage: b[f] [arg] # Get/Set block size
| b 33 set block size to 33
| b eip+4 numeric argument can be an expression
| b display current block size
| b+3 increase blocksize by 3
| b-16 decrease blocksize by 16
| b* display current block size in r2 command
| bf foo set block size to flag size
| bj display block size information in JSON
| bm 1M set max block size
The b command is used to change the block size:
[0x00000000]> b 0x100 # block size = 0x100
[0x00000000]> b+16 # ... = 0x110
[0x00000000]> b-32 # ... = 0xf0
The bf command is used to change the block size to value specified by a flag. For example, in symbols, the block size of the flag represents the size of the function. To make that work, you have to either run function analysis af (which is included in aa) or manually seek and define some functions e.g. via Vd.
[0x00000000]> bf sym.main # block size = sizeof(sym.main)
[0x00000000]> pD @ sym.main # disassemble sym.main
You can combine two operations in a single pdf command. Except that pdf neither uses nor affects global block size.
[0x00000000]> pdf @ sym.main # disassemble sym.main
Another way around is to use special variables $FB and $FS which denote Function's Beginning and Size at the current seek. Read more about Usable variables.
[0x00000000]> s sym.main + 0x04
[0x00001ec9]> pD @ $FB !$FS # disassemble current function
/ 211: int main (int argc, char **argv, char **envp);
| 0x00001ec5 55 push rbp
| 0x00001ec6 4889e5 mov rbp, rsp
| 0x00001ec9 4881ecc0000000 sub rsp, 0xc0
...
\ 0x00001f97 c3 ret
Note: don't put space after ! size designator. See also Command Format.