Using large DFT for OFDM to avoid the guard interval In OFDM, we usually insert the guard interval to avoid the inter-symbol interference at the expense of achieved data rate. that guard interval can be either zeros or CP guard interval, which it must be longer than the channel delay.
I am wondering, what's about using long symbol to avoid that guard interval ? for example instead of having number of subcarriers $N= 64$ or $N = 1024$, we can use $N = 5120$, what's the impact of using a high number of $N$ ??
Sure,  you can !
Practically, some systems are using OFDM system with number of subcarriers 4096, but the PAPR will be an issue in that case, and if your channel is fast-fading, the time-domain equalizer will be high-complex since one-tap frequency-domain equalizer will be useless.
Usually, choosing the length of the symbols depends on the channel delay, e.g., if your channel delay is 128,  you must use at least length of symbol 1024 and so on.