Of course there's always the chance that I'm a lousy writer, but how would they know? Odds are they never even read the synopsis.
The bigger question is, how would
you know?
And the answers are:
1) Beta readers, and/or
2) A novel workshop, and/or
3) Show Your Work on a forum here for a critique.
Just to give you a clue, standards have changed. 50-60 k used to be fine for fisrt novels, and people started getting twitchy as you pushed past 80k. There are now some publishers who won't look at anything shorter than 80k. Nowadays
60-80 k: reasonable, maybe too short for some genres
80-100: the sweet spot for most
100-120: a "big" book for most debut novelsits
120-180: Stephen King country, sketchy for most debuts, but but okay for some fantasy novels
180k+: Well, it's not impossible. And neither is winning the lottery. (See ORION's recent novel. No, it's not 180k+. It's about winning the lottery.)
The underlying problem is assessing how "big" your story is. Is it a 100k sotry massively overpadded, or do you really have a trilogy that needs to be fleshed out? The only way to tell is have people read it...
And--let me put on my asbestos suit here, as I'm probably going to get flamed for saying this--you don't
have to tell agents how long it is in the query. If they read a partial and they are desperately, achingly passionate to see more after reading the first three chapters, you have learned something very important--and they may help you decide how to parse it. On the other hand, if they aren't gripped by the first three, it's quite likely you need to haul out the old machete...