ClientID	ProductName	AvrageChange
100		bfcxb		1.12
101		tkczfbag	0.7      

ClientID        ProductName     Size    MeasuredDate
100             bfcxb           48.38   2/1/2010
100             bfcxb           49.65   3/1/2010
100             bfcxb           50.77   4/1/2010
100             bfcxb           51.12   5/1/2010
100             bfcxb           35.12   6/1/2010
100             bfcxb           36.87   7/1/2010

101             tkczfbag        2.06    7/1/2010
101             tkczfbag        1.36    6/1/2010

create table ejb
(clientID int
,ProductName varchar(10)
,size float
,Date smalldatetime)

insert into ejb
select 100,'bfc',48.38,'2010/01/02' union all
select 100,'bfc',49.65,'2010/01/03' union all
select 100,'bfc',50.77,'2010/01/04' union all
select 100,'bfc',51.12,'2010/01/05' union all
select 100,'bfc',35.12,'2010/01/06' union all
select 100,'bfc',36.87,'2010/01/07' union all
select 101,'bdd',2.06,'2010/01/07' union all
select 101,'bdd',1.36,'2010/01/06' 

--(8 row(s) affected)

select
x.clientID
,avg(DIFF) as avg_DIFF
from
(
select 
--	e2.size
--	,e2.date
--	,e1.size
--	,e1.date
	DIFF=e2.size - e1.size
	,e1.clientID
from ejb as e1
join ejb as e2
on e1.clientID = e2.clientid and e1.date = e2.date+1
) as x

group by x.clientID
order by x.clientID

select ClientID,ProductName,round(avg(avgs),2)AverageChange
 from
(
select *,(Size-(select Size s1 from yourtbl t2 where t1.MeasuredDate=DATEADD(dd,1,t2.MeasuredDate) and t1.ClientID=t2.ClientID))as avgs from yourtbl t1
)t 
where avgs>=0 group by MeasuredDate,ProductName