Bonnie, what Dave says is correct. I hope you are maintaining a personal database which you sync with Ancestry because that will make it so much easier for you to created small GEDCOM files to upload.
WikiTree does not populate the tree automatically with your GEDCOM. Instead, you will have to review a report called GEDCOMpare and either accept or reject all potential matches between the individuals in your GEDCOM file and those who may already have profiles on WikiTree. This is to prevent creation of duplicates. It would take a very long time to process a GEDCOMpare report based on a "huge" (as you put it) GEDCOM file.
Once you have processed the GEDCOMpare report, you may begin to add, one-by-one, any individuals from your file which did not have a profile already on WikiTree. You will be responsible for "cleaning" the profile created by removing "GEDCOM" junk, formatting a biography and the sources, if any. Remember, every profile needs to have one or more valid sources.
To get started, see this page...Special:UploadGedcom But, please, before that, also read other help pages on GEDCOMS such as Help:GEDCOMpare and Help:GEDCOM-Created_Biographies