I've been biting my tongue....
The size of the magnet is not the only variable here, and does not guarantee success.
Type of reed switch, placement/alignment, distance from the switch to the magnet, polarity "exposed" to the reed switch, are all variables in this working.
You stated you have a tender (no brand or model given) and said you have no LGB magnets (so one could guess it is LGB)
So LGB magnets will probably work, if it is indeed an LGB tender, and in good shape.
Otherwise, you may be experimenting.
As Peter suggested (I'm reading a bit more into what he stated literally), if you buy a "set" of magnets and switches from the same vendor, it is much more likely to work.
Greg