Is RIJNDAEL encryption safe to use with small amounts of text given to users?

I am thinking about making the switch to storing session data in encrypted cookies rather than somewhere on my server. While this will result in more bandwidth used for each request - it will save extra database server load and storage space.

Anyway, I plan on encrypting the cookie contents using RIJNDAEL 256.

function encrypt($text, $key) 
{
    return mcrypt_encrypt(MCRYPT_RIJNDAEL_256,$key,$text,MCRYPT_MODE_ECB,mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256,MCRYPT_MODE_ECB),MCRYPT_RAND)); 
}

Which in use would produce something like this (base64 encoded for display)

print base64_encode(encrypt('text', 'key'));

7s6RyMaYd4yAibXZJ3C8EuBtB4F0qfJ31xu1tXm8Xvw=

I'm not worried about a single users cookie being compromised as much as I am worried that an attacker would discover the key and be able to construct any session for any user since they know what I use to sign the data.

Is there a way I can verify estimated cracking times in relation to the parameters used? Or is there a standard measure of time in relation to the size of the text or key used?

I heard someone say that the keys needed to exceed 256bits themselves to be safe enough to be used with RIJNDAEL. I'm also wondering if the length of the text encrypted needs to be a certain length so as not to give away the key.

The data will generally be about 200 characters

a:3{s:7:"user_id";i:345;s:5:"token";s:32:"0c4a14547ad221a5d877c2509b887ee6";s:4:"lang";s:2:"en";}

So is this safe?