The amplicon size depends on the primer set used in the studies. The expected amplicon size can be calculated by running a digital PCR (see this forum post). For example, if you're using V4 primers the expected amplicon size should be ~252 nt; if using V3-4, the amplicon size could be ~450 nt.
The easiest way would be asking the sequencing company for it. They shall have good answers. Alternatively, you can grab all the commonly used universal primers for the 16S rRNA gene and blast them against your raw fastq files following the method I linked in the previous post.
The DADA2 won't work properly if primers are not removed from your reads, e.g., losing the majority of reads during the chimera removal. See the DADA2 tutorial for more details.