Close though requests aren't made up to 'codeword size' but in mini-slots, and one of the parameters in the upstream MAP is mini-slot size in 'ticks' of 6.25us. The modem asks for as many mini-slots as it needs then receives either a grant for those or a partial grant for some of them with the rest fulfilled when the CMTS can, which happens under congestion conditions. The amount of mini-slots a modem can request at once is set by the maximum upstream burst parameter, which you can probably read off your modem's diagnostic pages. They're allowed enough mini-slots from a single request to transmit that amount of data. Note that modems can, and do, encapsulate further requests for grants along with their data when transmitting across multiple bursts to avoid going through the contended mini-slots again to make their requests.
Concatenation, which you'll see in DOCSIS 1.1 and above and some 1.0 hardware, is also useful, it allows multiple small frames to be placed into a single burst request which seriously increases the packets per second as the modem doesn't have to go through the request-grant-transmit cycle for each and every datagram. Very relevant for very high frequency but small frame size traffic such as gaming.