-eres this setting is related to Ethereum mining stability. Every next Ethereum epoch requires a bit more GPU memory, miner can crash during reallocating GPU buffer for new DAG.
To avoid it, miner reserves a bit larger GPU buffer at startup, so it can process several epochs without buffer reallocation.
This setting defines how many epochs miner must foresee when it reserves GPU buffer, i.e. how many epochs will be processed without buffer reallocation. Default value is 2.
If we use big eres, will that affect the mining speed? Like bigger DAG?
-eres does not change mining speed at all. It just changes the buffer size. So if you don't have any problems with GPUs you can set "-eres 5" to make miner use the same buffer for five epochs (25 days). By default -eres is "2" so after 10 days miner will have to reallocate GPU buffers and may crash.
Buffer size itself is not related to the mining speed. Mining speed is slower with every next epoch because the data size in the buffer is growing.