Use parliamentary procedure, for example. One party, not wanting the other to pass something, could simply debate it endlessly taking all the time they have on the floor, one after the other. They could by sheer numbers attach riders and other nonsense to the bill overwhelming the original intent. Imagine some committee with 50 members split roughly in two. A quorum requires, let's say 60% present. The objecting party doesn't show up. Or, they do and endlessly add crap to keep things from happening. Much easier with bigger numbers. Or a Congressional hearing that drags on for weeks to give each member their time to badger witnesses...
Then you have to office and house all these representatives. In the 60's they all need phones, mimeographs, and other office equipment. They take up a huge office building apart from the Capitol building, or the Capitol building has to expand and expand. Getting a quorum to even vote on something could become difficult or impossible. They all need staff too.
Then there's the problem of some state having so few people that it has 1 or 2 representatives out of a 1000 or more. They become irrelevant and their state likewise becomes irrelevant. It has no voice.
It would be an endlessly expanding bureaucracy of inefficiency and nonsense.
They likely couldn't govern at all.