You can get outside of a reasonable range for some things (adding a 2000L sump or using a skimmer rated for 543523432L), but oversizing isn't really too much of a concern for me. I'd rather over-size than undersize, but still stay within a reasonable range of what I am trying to accomplish. Like I wouldn't get a reactor rated for 2000L, but maybe one for 500. Some things you don't want to oversize too much (especially protein skimmers) since they do not perform as well if they don't have the correct conditions.