Good on you! Persevere and you will succeed!
I use several sizes, depending on application. I advise to screw in straight to get the box square and level, then fix with screws 'on the tosh'.
For me, if smaller diameter screws are used and fail to hold, larger diameter ones and likely longer will replace them!
Certainly, a waterproof glue in the joints will make the boxes more durable. Until competent, dry assemble first, then apply glue at final assembly.
Assembling your boxes on a flat surface on a square sheet such as a queen excluder can make things easier.
Don't buy cheap screws (or cheap driver tips, for that matter) as there is a risk of spoiling the drive slot (I invariably use posidrive countersink screws). I prefer Reisser screws as they are reliable and long lived.
Less than 50mm are likely to be required for securing the side bars to the hive wall from the inside, again 'straight in' pulls the bar tight to the wall and 'on the tosh' makes them very secure (but may need countersinking to hide the heads). In fact I would advise countersinking all the screws, as that avoids distorting the wood, until you are experienced.
Regards, RAB