Let's say I am considering buying some on an underlying that I think will recover significantly between now and January 2026, the longest dated option available.
If I look at the Interactive Brokers option chain, I see for example right now:

Underlying current price: $28

January 2026 Calls:
Strike: $50
Option Open Interest: 546
Volume: 28
Bid Size: (varies rapidly from 200 to 800)
Bid: $5.75 (varies slowly)
Ask: $6.20 (varies slowly)
Ask Size: (varies rapidly from 1 to 600)

A couple questions:
- are the bid and ask sizes in # of contracts? (I assume yes, though the numbers are quite large compared to the volume traded)
- the Volume is not changing, even though the bid/ask and bid/ask sizes are changing. Does this mean there are no executions?
- If I want to build a position of, say 15 contracts, how would I do that? Enter orders for a couple contracts at a time, with price midway between bid and ask say?

Mark