I've flown loads in this pregnancy, and would still be flying now if I was allowed (grounded at 34 weeks..)
I flew long haul to Canada at 6 months, which was only a drag because of the BA strikes which meant I had to transfer at amsterdam and was traveling alone, so no one to carry stuff for me. I wasn't huge at that stage and was absolutely fine. From my experience and the experience of friends, if you're having a fine time at 4 months, you'll prob still be having a fine time at 6 months (and 8 months hopefully!)
I've been back and forth to Ireland and Spain since then, both for work and hols. I was 7 1/2 months in Spain - still not huge, but def on the bigger side, but absolutely fine both traveling and sightseeing - I happily climbed up steep hills and stuff that the slightly overweight people around me were having problems with!
And I also did some quality sunbathing - top advice - I slathered sunblock on my bump but still got a bit burnt, so really slap on the factor 40!
I got a 'fit to fly' letter from my docs for flying after 28 weeks (the cut off for most lower budget airlines) but never got asked for it.
I did always ask for an aisle seat when flying, but I've not been that troubled with needing the loo all the time anyway, so prob would have been fine if I hadn't had an aisle.
My advice, if you're feeling positive and healthy, then just go for it. If you enter into these things thinking 'I'm going to suffer' then you do, if you have a 'this is going to be great' attitude, then it will be great.
Have fun!