A couple of years ago I purchased a 5-stone ring from
Whiteflash (I'm very happy with it, search on my user name for 5-stone thread photos that are in addition to the one shown in my avitar). If you put in the search term hawaiianorangetree and see her thread for her 5-stone ring you will more photos of a really nice 5-stone ring (hers from
Brian Gavin). Both, I believe, good examples of trellis design.
As for size, one can almost never get too large a diamond. I wanted a nice graduated look for my ring and ended up with a gradation of .25, .41, and 1.01. My wife's ring is spectacular (as I'm sure, in person, hawaiianorangetree's ring is) with five A-Cut-Above diamonds. The gradation worked out as I had hoped.
a) However, if I had it to do over again I might have gone with 1.25 carat center stone, forget about the smooth gradation!
b) You "must" get super-ideal-cut diamonds or very nearly close. Regardless of what I-D Jewelry calls their premium cut diamonds (as opposed to
Whiteflash A-Cut-Above or
Brian Gavin's Signature), I know they can supply them - and they make all the difference.
c) There seems to be a price/carat price jump when you get to or near 0.5 carats. I would suggest asking the helpful people at I-D Jewelry about a center stone in the 0.40 to 0.43 carat range, as you should still be getting a lot of bang for your extra buck, in that size range.
My advice: It is pretty easy to search on-line on the web sites for any of the three vendors I've mentioned and see how pricing moves from .30 to .35 to .40 carats. Yes, .40 carats costs more than .35 carats, but I don't think you'd regret the extra expenditure a year from now, even for the small millimeter increase in diameter - you would have gotten the largest center stone you could "afford"
I know there has to be a stopping point somewhere when wondering how much "more" to spend, or not spend. I think the .40 carat center stone would be a good value if you could stretch the budget a little (more

).