I have a pair in a 100 gallon, so 140 should work as well - at least for a while until they outgrow it (may take years). Important is also a good rockwork that allows the smaller one to get out of sight quickly.
Here 2 videos of the tank to get an idea what works:
Front view:
<iframe width="560" height="315" src="https://www.youtube.com/embed/wuRw2mB2oiM" frameborder="0" allowfullscreen></iframe>
Side view:
<iframe width="560" height="315" src="https://www.youtube.com/embed/RIJj9l7AEhM" frameborder="0" allowfullscreen></iframe>
My two are 95 mm (3.7") and 80 mm (3.2") in total length. The larger one is by now done with loosing its eyespot (color change from juvenile to adult), while the smaller one is close to loosing its eyespot. Both are still immature subadults with tiny operculum spines. Both eat pretty much everything I throw into the tank: frozen, pellets and flakes (haven't tried bread yet but I would expect them to eat that as well

)
With your tank size you could probably go with 120 mm (~5") for the larger (male) and 100 mm (~4") for the smaller (female).
Sexing is usually quite easy with fully mature adults at sizes well above 150 mm (~6") (
see here)
This should be a mature male:

Size (mm): 147 SL, 183 TL (7.2" TL)
Locality: Egypt, Nuweiba, Gulf of Aqaba
And this a mature female:

Size (mm): 130 SL, 162 TL (6.4" TL)
Locality: Djibouti, Maskali Island,
Note the much longer and more massive operculum spine on the male and the slightly more stretched body shape.
Fully mature males have operculum spines that are about the length as the distance from snout to operculum edge (or the base of the spine).
The operculum spine of mature females is about half that length.
With around 150 mm (~6") TL individuals the relative body length and operculum spine length of the individuals is more important: at about the same body height (not length!) one needs to be longer and have a longer spine than the other. The female should always be smaller than the male.
Though larger individuals do not always adapt well to captivity. Alone for that reason I would rather go with smaller individuals.