Dynamic spectrum access (DSA) is the key feature of cognitive radio (CR) networks, but it also poses many new challenges on the medium access control (MAC) design. One of key challenges is the fact that the secondary CR users can only borrow the licensed spectrum from the primary users for a short period of time. Hence, unlike many available multi-channel MAC protocols for ad hoc networks where throughput is the main performance issue, the DSA protocols in CR networks shall place more emphases on the access latency. Hence, one fundamental issue arises: how can the spectrum be dimensioned for control channels in order to minimize the access delay of DSA protocol in CR networks? In this paper, we provide a comparative study in an analytical manner on the latency performance of two DSA protocols: 1) dedicated control channel, and 2) embedded dedicated control channel approaches. Our results show that an optimal ratio of the control channel bandwidth over the total channel bandwidth can be found to minimize the latency of DSA with dedicated control channels. However, the delay performance of DSA with dedicated control channels is more sensitive to the variations of the data lengths than that of DSA with embedded control channels. Hence, we conclude that the way of dimensioning the spectrum for control frames for DSA in CR networks should be adaptive to the variations of the traffic characteristics and the number of users.