By common do you mean the senegal bichir? Well in anycase the senegal in my experience are grow slowers than other bichirs but that's just my experience. But I will give you a good example of their growth rate.
I use to have one last year that I bought at about 3"-4" inches. It grew to about 6"-7" inches took about 8 months to do so. A friend liked it and so I gave it to him as a gift.
I literally bought an endli bichir at the begining of this month March 2011. It was 1.5"-2" and about the size as a pencil around or smaller. No Joke, It is now 7" inches long it's been 3weeks only. So they grow pretty quick.
I went out and bought another senegal earlier this year in Jan it was 3"-4"inches too when I bought it and was the size of a pencil around or a lil bigger. It is at the moment 4.5"-5" inches only. Big difference.That's a good 2.5-3 months older than the endli.
I personally thought it was a fluke for the endli I just bought earlier this month. I went out last week and bought another at 1.5'-2" inches endli guess how big it is as of right now? It is at 4"-5" inches right now. They get real big real quick.
As for the Senegal species maybe it's just me. I've had one a while back and it didn't grow much and I bought one earlier this year and my endli's out grew it with lightening speed. Oh right I have had ropefish/reed fish too and they and like the senegal not the best growers in my opinion. So That's my personal experience hope it helps.
you'll definitely have to upgrade the tank if you plan on getting an endlii. and since its full length is bigger than what Loves severum suggested, you will have a quick growth rate.