sump/refugium for 180gallon

Hi,

I have a 180 gallon and i'm new to marine and was just wondering if a 20 gallon high or long is good for a sump which i will have one chamber with protein skimmer the middle with refuguim and then the last return. My space under the tank is very limited and low with a height of about 19" to play with as the 20 high is already a tight fit. Also any recommendations for protein skimmers? The tank will be a FOWLR

Thanks much appreciated