Seating chart for about 20 tables

I don't think 130 guests is a huge wedding. It's kind of in the middle of small and big. But if you do the math, your numbers make it so that each table will only have 6 or 7 people sitting there, whereas most of the venues I've talked to seat about 8-10 people per table. Is this something you are doing purposely (spreading guests out) for comfort, or is this how your venue does it? If you put 8 people at each table, you'll have approximately 16 tables; if you seat 10 per table, that's only 13 tables, which seems about average.

However, to answer your question, you don't have to assign exact seats, just tables. Regardless of the actual physical layout of the tables in the room, you can basically list your tables, and decide which guests will sit at each one. Example: (for 8 per table)--Table 5 has: uncle john and aunt susie, cousins jimmy and julie, aunt and uncle #2, and grandma and grandpa. That's 8 people seated at Table 5. Just do that for every table. When it gets closer to your wedding (or even now if you're anxious about it) you can talk to your venue about the actual layout of tables. They probably have a lot of experience at this, and can tell you what layout usually works best for the room. They may even have a layout map, showing the usual placement of tables for a wedding reception.

I'm having about 130 guests and plan to have escort cards displayed on a table during cocktail hour. They will be set up in alphabetical order and guests will grab them while they are mingling, before going into the ballroom.

I would say anything under 150 is a pretty normal size wedding, under 100 is smaller, 150+ is larger. So 130 doesn't sound super big. We had 125 and we just did the seating chart like this. We assigned individually seats, so place cards were at each spot. We easily could have fit another 10-20 guests this way.

Well, there are different ways to do it. Some people here have suggested escort cards. I think I'm just going to do like larger cards (like 5x7), one for each table, that has the names of every person assigned to that table. They will be assembled onto one big posterboard-style sign near the entrance to the reception.