try the Nes,Snes ,and or the PlayStation games

PlayStation has a lot of games and all of them are pretty small size, also SNES and GBA...

Here is a link you can download some emulators and ROMs, hope it'll help ya

CoolROM.com - N64 ROMs GBA ROMs SNES ROMs NDS ROMs GBC ROMs NES ROMs MAME ROMs PSX ROMs Gamecube ROMs ISOs and Emulators

What do you consider small size? A few MB, or is a GB allowed?

Well, old games is a big term.
Depends on what you're into really.

You have early DOS games, which have everything from RPG, Platform, Shooters, Point and click classics ...
If that's what you're looking for , you might want to download DOSbox and learn how to use it a bit.

As for the games:

1. RPGs: -Elder Scrolls Arena and ES Daggerfall, predecessors to skyrim so they can't be bad . Then you have Might And Magic: Heroes 1 and 2, they have a kind-off a strategic feel to them but still they are really fun if you are into that kind of games.
   
2. Platformers: There are just a lot so im just going to list some of my favorites. Jazz The Jackrabbit, Duke Nukem 1 and 2, Commander Keen series (especially games from part 4 and beyond) , Prince of Persia and Prince of Persia2 :Shadow and Flame...
   
3. Shooters: There is not too much of them this early in gaming , there are such amazing classics as Doom series, Wolfenstein, Duke Nukem 3D ,Hexen...Even if there are so few and they don't look as much at first glance ,they are amazing must try classics.
   
4. Point and click adventures : Ahh,one of golden genres of this era. You have games as Secret of Monkey Island which is more of an interactive story with puzzles than a game, but still it's funny and has that 80ies-90ies humor vibe to it. Then another one is Indiana Jones and The Secret of Atlantis, it's another awesome game with amazingly beautiful graphics for it's time...But some times the puzzles get too hard and you tend to get stuck so don't hesitate to use a walk-trough if you do get stuck in one place. Also you might want to use SCUMMVM to play these games since it is easier to use than DosBox and gives you better emulation.
   

PS. There are some RTS games from this era but i don't rly know how to emulate them , like Age of Empires (thou the second one got a HD remake) and C&C Tebirium Sun.

Ahh this is going to be a long reply...im going to shorten it as much i can here,soooo off to consoles.

NES had a lot of classics, but it really shines for most in Platforms, like Super Mario series, Kid Icarus...There are some other games, you might want to explore it on youtube there are a ton of channels that explore these games.

Snes, i would recommend lots of awesome RPG games like ,Final Fantasy ,The Secret of Mana, Super Castlevania 4, Mario Kart is awesome for 2 players...

N64 , im not really informed too much about this one, i know stuff like Banjo Kazooie , Conker's Bad Fur Day (<-- Pewds actually played this one), Mario 64, Mario Kart 64.......

Sega Genesis/Mega Drive(in Europe) again has a lot of awesome ones, like the SANIC series (gotta go fast) ,then there are like EarthWorm Jim 1 and 2, Out of this world...

PS1 has a lot of good ones , like Crash Bandicoot , Final Fantasy, Tekken 3...

That's about it, it's games that i found fun, but honestly you will have to explore the retro game world a bit more yourself in order to find the games more to your liking, there are tons of channels ,top 10s, websites...

Anyway, hope i helped

hey pewds its you

Maniac Mansion: Day of the Tentacle is pretty much the best retro game that pops into my mind.
Other than that you may wanna try Chrono Trigger or the Megaman series.