It has been a while, but I was having the same issues as you.
After much digging, I found that the wonderful world of microsoft explains:
To avoid errors during importing, ensure that each source column contains the same type of data in every row. Access scans the first eight source rows to determine the data type of the fields in the table. We highly recommend that you ensure that the first eight source rows do not mix values of different data types in any of the columns. Otherwise, Access might not assign the correct data type to the column.
Apparently, this means when appending an excel file to an existing table, even when columns are formatted and saved as memo fields, that if all 8 of the first rows in the excel file are less than 256 chars, Access assumes you actually meant to specify text, thus truncating the remaining rows after 255 chars. I have performed several tests placing "dummy" rows within the top 8 rows, and each triggered the import of more than 255 chars.
Now, if you import to a new table, the wizard allows you to pick all of the formatting options.
Importing to a new table is convenient if you are okay with overwriting all of the data already in the table. However, if you truly need to append, I would suggest importing to a temporary table, then appending from there. An easy way to do this is to save a import then execute it from VBA, like Elliot_et_al wanted to do. You could then also run the append query in VBA as well. If you set up your tables correctly you may be able to get away with
INSERT INTO [MyTable]
SELECT [MyTable_temp].*
FROM [MyTable_temp];