Can't say about the size, but you could try and check for the last entry or the amount of entries in the dump and then check for the current last entry in your database. This might help you determine the time the import will take till it finishes.
The size of the imported data might even get bigger than the uncompressed 7GB, since the indexes are usually not contained in dumps but get built on insert.
As a sidenote: This is also a way to speed up the import itself: drop the index during import and rebuild it later on, this helped me several times to speed things up.
I think I have a plausible explanation you find very intriguing.
When you loaded the MyISAM table, only one index got loaded into the .MYI file in a lopsided manner.
Which index would that be ? THE PRIMARY KEY. Why ?
Indexes usually use BTREEs. They are designed to collect keys into a single BTREE node and balance the BTREE internally until the BTREE is full. On the insert into the full node, the full tree node is split and the keys inside are divided.
Most often a BTREE node will split is when you load the keys in order. For proof, look at the worst case BTREE --- the balanced binary tree.
I have several posts in the DBA STackExchange where I mention how a balanced binary only has node with one key. I also mention how a leaf pages must rebalance about 45% of the time when inserting a new key. Here are those posts:
My working theory is that random inserts into the table may actually make the .MYI smaller by prevent tree node splits as much as possible.
In order to make this happen, you will have to something quite unusual. Ready for this ?
ALTER TABLE generic_dummy_table_name ORDER BY datetime;
or
ALTER TABLE generic_dummy_table_name ORDER BY user,datetime;
That right, you can reorder the physical rows of the table. This may make the indexes a different size by inducing BTREE splits on another index.
Try reordering the table. Then, mysqldump it and reload it. You may find that the .MYI may get smaller or bigger. You cannot really predict it.
Next time, try reloading the MyISAM table with a very large bulk insert buffer.
Just run
SET GLOBAL bulk_insert_buffer_size = 1024 * 1024 * 1024;
then connect to mysql and reload the mysqldump.
GIVE IT A TRY !!!
Best Answer
MySQL has huge limits -- many terabytes.
You may run out of disk space.
Your app code may run out of RAM. But only if you foolishly fetch all the rows at once. Instead, use SQL to filter/summarize the data you fetch.
But otherwise, don't worry.