Well, every link transmits only either 0 or 1 at any specific moment. So if your router connected to 2Mbit/s link receives first 250 bytes packet, and assuming that it doesn't have anything at the moment to send, it will pass the 250 bytes into the interface driver, and then serialize it (meaning put 0s and 1s) onto a link.
The next packet, even if it's just milisecond later, will wait for the first packet to be transmitted. This may incur delay in sending (alone) of 1 milisecond, but that's just theory - in reality, there will be some additional, minimal delay depending on the way your router works.
Take a look here for additional writeup.
There are techniques to fight with serialization delay (take a look here), but in todays networks and with your 2Mbit/s link, you may find them unnecessary. You shouldn't have to use them, as even with 1500 bytes segments, with proper priority queue implementation, this link shouldn't incur delay that's visible for typical IP-based applications.