i am planning to launch a weblog system (web based), i am expecting to
have 150,000 subscribers, and it will be a free weblog.

My questions is:
1- How much space is ideal for a subscriber (in MB)? 
2- Since i dont have my own servers i will be looking for a hosting
company andi would like to know What server specifications i should
have for my weblog system (size, bandwidth...etc)
3- And how much it will cost me to host my weblog system?

1. It depends on your users and their usage, However to put them all
on one dedicated server and not go over 80GB of hard disk space their
limit will be 0.53MB (~ 530KB), I recommend you to make it multihome,
So you can host some of them on one server and others on another
server.
2. Again it depends, but on your desired weblog system, There are lots
of them writen on PHP and MySQL such as geeklog.net, In that case you
will need a Dedicated Linux server with lots of bandwidth (more than
300GB/mo). Size will be your total expected, lets say 100GB.
3. Such server at a reliable data center will cost you at least $150/mo

You may also start from a cheap hosting account, start and then
upgrade to higher values as your site grown.
http://www.dayanahost.com/ has some good prices. I recommend you to
start with them.

farmal

The disk space requirement depends on number of users and daily posts.
With 150K users 80-120 GB disk will suffice.

The web server must be able to nicely carry your traffic load. Minimum
configuration I recommend will be:

PIV 3.0 Ghz HT
80-120 GB Disk x 2 (backup)
1 GB RAM
1000 GB Bandwidth/monthly
2 IPs

You can get above mentioned configuration for $100-$150.

Check www.hostbreak.com as they are reliable and I have heard nothing
but good about them. Their staff can assist you further.

azizparacha

Please visit http://www.trade-task.com ,
if you order any hosting package you can ask them for establishing a
weblog system, and they do it for you free.