It really depends on what the game is trying to achieve. The universal rule for me these days, though, is no filler combat. That doesn't mean no trash mobs - a combat focused game like WL2 can have a lot of encounters, but it gives you a way to skip random ambushes, and each progression in enemies brings a new tactical element, whether that's spreading out once red skorpion grenadiers show up, breaking your 'get cover and focus fire' habit to ensure that you have guys chasing down multiple enemies with rocket launchers, or basing your tactics around taking out the energy gun synthetics in the knowledge that their squad mates can't do a lot of quick damage against your heavily armoured end game party.
Similarly, BG2 makes the most out of its enormous scope by dividing it into a huge number of bite sized dungeon crawls that you can knock off in one play session. Most of them only through a couple of standard 'mooks attack' at you before mixing it up - orcs, then orcs with kobolds shooting fire arrows, then a gauntlet with the kobolds firing at you through a protective grate. By switching to its '4 combat zones' model, Bioware made much smaller games feel like much more of a slog.
Occasionally a long slog can have a thematic effect. Not a crpg, but FEAR used its length (and some simple but effective design work in having the light outside track the transition from dusk to dawn) to create a 'this has been a fucking long night' feel to it - 40 miles of hard road.
So i'd put encounter design and a macro plan for the games 'feel' ahead of a general rule on length. The old design goal of 'large game, short play-sessions' seemed to fall by the wayside once open world crpgs and procedurally generated content took off, even in games that have neither of those things.
Conversely, if your game isn't about combat, don't put trash mobs in just to pad the time between plot points. That Bioware writer who wanted an option to skip combat deserved some of the flak that resulted, but she isn't totally without a point. Much of the combat in Bioware games serves no purpose, and even the most hardened combat-fag ends up clicking through while waiting for something to happen, where that something is inevitably plot progression. Sure, ideally you'd fix the damn combat, but realistically developers are always going to have a particular aspect of the game that they prioritise, saying 'this is what our game does well', while other factors are left simple or skimmed over. This is the great flaw with Bioware design - if their selling point is 'let's tell a cinematic summer blockbuster', why the fuck would you make that less than 5% of the play time, while the other 95% is spent grinding through trash mobs? It really would be better to give the option to skip that shit, and maybe it would force Bioware to actually come up with the quality of story and C&C to which they so clearly aspire.
Even PS:T could have trimmed its already small amount of combat. The heart of the game is as an adventure /crpg hybrid, where combat exists either to give character to an area, to show TNOs transition to near godlike power (to then set the stage for realising how useless that is to him), and for major plot critical encounters with very well developed NPCs. Yes, again the ideal solution would be to fix the damn combat, but I'm not sure that time and budget would have allowed that to be developed simultaneously with the games experimental explore-through-dialogue focus. And it might have even been detrimental by pulling the players' focus away from the heart of the game (same if FO had been full of tactically challenging encounters, instead of flavour and a few memorable set pieces - sure, it could have been improved, but Wiz8 levels of tactical combat would have destroyed the game's feel).
If it went the other way, and limited the combat to plot/character/setting relevant encounters, it would have enhanced the sense that combat is just one of many tasks around which you can build your character. Even the combat itself may have benefited, as the designers could craft the combat around fewer, more interesting, encounters.
TL;DR, length isn't an issue, it's the way it's divided up, plus encounter design and synergy with the game's intended feel.