Gameplay-wise, anything more than 20 planets per player (that's 200 planets total for a 10 player scenario) doesn't play very well.  The vast majority of your time will be spent covering ground, either before you meet the enemy or after they're defeated with no hope of recovery.  You can always create such maps using GalaxyForge, but I've always found that the ideal map size is in the range of 5-15 planets per player.

Most of my games when finished took 12 to 16 hours. I play those huge maps with just four different players. And for some reason the maps I make with the forge dont work so yeah.

Using the In-game map maker rarely provides issues.

Err, well the instant defeat is usually from not giving all players a starting planet or home world. Even if you set up a desert or terran "home" planet, you need to give it to a player in the ownership tab and then set homeworld to True.

By not showing up I meant it wasn't appearing in your galaxy forge tab, but if you know it is giving you instant defeats, clearly you can select it.

The one I made for the Distant Stars Universe has 429 objects currently and we haven't even populated half of the star systems...  It's a torturous map to play on and it's only getting worse as the lore expands.