wget没办法直接限制下载文件大小，而curl有这个功能，所以可以用curl来做文件下载
–max-filesize 9999 文件要小于9999 bytes， 文件大于此致会返回63错误，而不下载文件
wangxh@mac : ~$ curl 'http://www.baidu.com/img/10.23_112a036972a74bbcb84b8957316c8aa3.gif' --max-filesize 9999 -O -v
* About to connect() to www.baidu.com port 80 (#0)
* Trying 119.75.218.70...
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0* connected
* Connected to www.baidu.com (119.75.218.70) port 80 (#0)
> GET /img/10.23_112a036972a74bbcb84b8957316c8aa3.gif HTTP/1.1
> User-Agent: curl/7.24.0 (x86_64-apple-darwin12.0) libcurl/7.24.0 OpenSSL/0.9.8r zlib/1.2.5
> Host: www.baidu.com
> Accept: */*
>
< HTTP/1.1 200 OK
< Date: Tue, 23 Oct 2012 14:45:03 GMT
< Server: Apache
< P3P: CP=" OTI DSP COR IVA OUR IND COM "
< Set-Cookie: BAIDUID=E8A14636EF42D5B73AB96A6B8E7F2D99:FG=1; expires=Wed, 23-Oct-13 14:45:03 GMT; max-age=31536000; path=/; domain=.baidu.com; version=1
< Last-Modified: Fri, 19 Oct 2012 08:31:06 GMT
< ETag: "3333-4cc65532c0680"
< Accept-Ranges: bytes
* Maximum file size exceeded
0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0
* Closing connection #0
curl: (63) Maximum file size exceeded
文件大于这个限制即可成功
wangxh@mac : ~$ curl 'http://www.baidu.com/img/10.23_112a036972a74bbcb84b8957316c8aa3.gif' --max-filesize 99999 -O -v
* About to connect() to www.baidu.com port 80 (#0)
* Trying 119.75.217.109...
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0* connected
* Connected to www.baidu.com (119.75.217.109) port 80 (#0)
> GET /img/10.23_112a036972a74bbcb84b8957316c8aa3.gif HTTP/1.1
> User-Agent: curl/7.24.0 (x86_64-apple-darwin12.0) libcurl/7.24.0 OpenSSL/0.9.8r zlib/1.2.5
> Host: www.baidu.com
> Accept: */*
>
< HTTP/1.1 200 OK
< Date: Tue, 23 Oct 2012 14:46:41 GMT
< Server: Apache
< P3P: CP=" OTI DSP COR IVA OUR IND COM "
< Set-Cookie: BAIDUID=D28F63CA8F4FACA8944261253F7A4992:FG=1; expires=Wed, 23-Oct-13 14:46:41 GMT; max-age=31536000; path=/; domain=.baidu.com; version=1
< Last-Modified: Fri, 19 Oct 2012 08:31:06 GMT
< ETag: "3333-4cc65532c0680"
< Accept-Ranges: bytes
< Content-Length: 13107
< Cache-Control: max-age=315360000
< Expires: Fri, 21 Oct 2022 14:46:41 GMT
< Connection: Keep-Alive
< Content-Type: image/gif
<
{ [data not shown]
100 13107 100 13107 0 0 82110 0 --:--:-- --:--:-- --:--:-- 168k
* Connection #0 to host www.baidu.com left intact
* Closing connection #0