Hello David
According to RFC 2332 which defines NHRP, the Destination Protocol Address (DPA) is:
…the protocol address of the station toward which a “request” packet is sent.
In other words, if you send a ping to 192.168.100.1 (the destination of the ICMP echo request) this is the address that appears in the field of the DPA. It is not the next hop address of the spoke, nor is it the tunneled address, but the destination of the actual transaction, whatever that may be. This is the case regardless of what phase DMVPN you use.
Concerning the prefix length, the same RFC says the following:
This field is message specific. See the relevant message sections below. In general, however, this fields is used to indicate that the information carried in an NHRP message pertains to an equivalence class of internetwork layer addresses rather than just a single internetwork layer address specified. All internetwork layer addresses that match the first "Prefix Length" bit positions for the specific internetwork layer address are included in the equivalence class. If this field is set to 0x00 then this field MUST be ignored and no equivalence information is assumed (note that 0x00 is thus equivalent to 0xFF).
This essentially means that the NHRP message pertains not only to the specific DPA, but also to the subnet to which that DPA belongs. The size of the subnet is indicated by the prefix length.
I hope this has been helpful!
Laz