How to set block size on Oracle 11 when creating a database instance

This db block size is causing our DB Buffer Pool to be constantly filled up (95%~100% full)

Your DB Buffer Pool should always be full unless you have recently restarted your server - Oracle will not purge blocks from the cache unless it has to in order to make room for other blocks.

This in itself is not an indication that changing the block size will improve anything. You need to know more about the cache misses.

If the OLTP side of your workload can query any part of your 5TB data, optimizing the buffer pool a little for it will probably not make much difference (but may unnecessarily slow down your big sequential scans that it was presumably optimized for)

On the other hand, if there is a small subset of the data hit regularly, and cache misses for the OLTP workloads can be substantially reduced by reducing the block size (which will depend on whether the reads are really scattered across blocks or not), then perhaps the benefits would be significant.

In the end you will have to test and find out :)

The best check for memory allocations is the buffer advisories. These estimate the impact on caching effectiveness of modifying the cache sizes, so you know whether you have the sizes of the PGA and SGA correct, and whether reallocating memory from one to another would be beneficial.