I prefer to use either a solo instrument or trio/quartet of some kind rather than an orchestra, and therefore, when the need arises to write something for orchestra, it's for a very specific combination (usually one that could barely be termed an 'orchestra'). For instance, I'm writing a commissioned piece at the moment that is for:
1 flute
2 clarinets (B flat)
1 trumpet (B flat)
1 tuba
strings
Pathetic.

The only other decent/good orchestral piece I ever pulled together was for:
1 piccolo
2 flutes
2 oboes
1 english horn
2 clarinets (don't remember the key)
1 bass clarinet (B flat)
2 bassoons
1 contrabassoon
3 trumpets (C)
timpani
xylophone
bass drum/triangle/cymbals
strings
I think, though, that my ideal orchestra (if I had to choose) would be the following:
1 piccolo
1 flute
2 oboes (or maybe just one)
1 piccolo clarinet
1 clarinet (A)
1 bassoon
1 contrabassoon
3 horns (F or possibly B flat basso)
2 trumpets (C)
2 trombones (probably both tenor)
1 tuba
timpani, xylophone, and several unpitched (the number should be out of proportion to the rest of the orchestra), maybe ondes martinot and/or theremin, flexatone (maybe in place of the xylophone)
small to medium strings
I mean, do you really need more than that? 15-16 winds, xylophone, optional electric instruments, and divisible strings can all carry out pretty much any kind of necessary stuff. To be honest, I know that I listen to a lot of Varese and Shostakovich, who both wrote for some of the largest combinations ever, but I am firmly an instrumental semi-minimalist in practice. (I mean, after all, there's quite a bit you can do with even a single piano, not to mention a string quartet!) And then there's also the issue of practicality, which is not 20 horns in any circumstances. The electric instruments are admittedly pushing it quite a bit, but rest assured that I've never actually written for any of them. 99% of my pieces are for piano or other tiny standard-ish chamber combination.
And thank you, Steltz, for giving us that advice! As a high school student myself, I can attest to the value of standards (or even small ones- my school orchestra last year had no horns but a good saxophone in place of the english horn, 2 good bassoons (and one bad one), and 4 trombones; this next year, we might have one horn and possibly the saxophone, but no bassoons; and the year after, it's looking pretty certain we'll have no oboes- a particular disappointment since the one we have now is literally better than many professionals I've heard). Another good reason to just write for a few people. I really like the idea of your 5-piece opera orchestra!