Screws do not need to be anywhere near as long as track pins. Do not use longer screws than you need. My 00 screws are the correct diameter for going through the factory holes in Hornby 00 gauge and similar. (not suitable for old Hornby Dublo Tinplate track.)
6mm screws are suitable for Hornby and peco going directly into a base board. If using foam underlay you will need longer screws according to the thickness of your underlay.
8mm screws are long enough for Fleischmann profi track
12mm will suit Bachmann e-z track and Marklin c-track
14mm / 16mm
My N screws are thinner than my 00 screws and are also suiatable for Hornby TT.
6mm screws are long enough to hold Feischmann N gauge, and Mini Trix etc and Hornby TT direct ot a baseboard.
8mm might be suitable if you are using 2mm cork on top of your base board.
10mm suitable for Kato N gauge
Note - Peco N gauge track - my N gauge screws are slightly too large in diameter for Peco N gauge track, for best results you will need to drill out the holes to 1mm, if you don't do this there will be some stretching of the plastic in the sleepers when the screws go through with resulting discolouration.
The 'Mini screwdriver' is a small (8cm long) very basic screwdriver. It's cheap for a reason. It is only suitable for 00 gauge 6, 8 and 10mm long screws.
This is a 14cm long screwdriver with swivel cap for more comfortable screwing.
15cm long screwdriver with swivel cap for more comfortable screwing. Only available for 00 gauge 6, 8 and 10mm long screws
15cm long screwdriver with ergonomic handle and swivel cap for more comfortable screwing. Far better than the others.